fix(eval/method): add missing method support for existing built-ins

These functions were ported with the vim-patch token, but didn't actually port
the method call support that was in their patches (method call syntax wasn't
ported yet).

Add the missing method call support and latest docs for:
 - assert_nobeep:
   https://github.com/vim/vim/commit/5b8cabfef7c3707f3e53e13844d90e5a217e1e84
 - buffer_name, buffer_number: (obsolete)
   https://github.com/vim/vim/commit/a8eee21e75324d199acb1663cb5009e03014a13a
 - charidx:
   https://github.com/vim/vim/commit/17793ef23aae0bc94539390ccfe5e63b0ad39ff2
 - flatten:
   https://github.com/vim/vim/commit/077a1e670ad69ef4cefc22103ca6635bd269e764
 - prompt_getprompt:
   https://github.com/vim/vim/commit/077cc7aa0e0c431e97795612374fe17fe7c88803
 - searchcount:
   https://github.com/vim/vim/commit/e8f5ec0d30b629d7166f0ad03434065d8bc822df
 - strptime:
   https://github.com/vim/vim/commit/10455d43fef041309ce0613fa792c635dd71e3a8
 - win_gettype:
   https://github.com/vim/vim/commit/00f3b4e007af07870168bf044cecc9d544483953
 - win_splitmove:
   https://github.com/vim/vim/commit/d20dcb3d011da6111153109f6e46fbd5c7fe9fb6

Also fix assert_beeps, assert_nobeep and getenv to accept exactly one argument.
Previously, they could erroneously accept one or more.

2 files changed, 25 insertions, 3 deletions

diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index 4b49cb6b6e..d1af4c4d72 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -3262,7 +3262,7 @@ char2nr({string} [, {utf8}])					*char2nr()*
 
 		Can also be used as a |method|: >
 			GetChar()->char2nr()
-
+<
 							*charidx()*
 charidx({string}, {idx} [, {countcc}])
 		Return the character index of the byte at {idx} in {string}.
@@ -3285,6 +3285,9 @@ charidx({string}, {idx} [, {countcc}])
 			echo charidx('áb́ć', 3)		returns 1
 			echo charidx('áb́ć', 6, 1)	returns 4
 			echo charidx('áb́ć', 16)		returns -1
+<
+		Can also be used as a |method|: >
+			GetName()->charidx(idx)
 
 chdir({dir})						*chdir()*
 		Change the current working directory to {dir}.  The scope of
@@ -4364,6 +4367,9 @@ flatten({list} [, {maxdepth}])					*flatten()*
 			:echo flatten([1, [2, [3, 4]], 5], 1)
 <			[1, 2, [3, 4], 5]
 
+		Can also be used as a |method|: >
+			mylist->flatten()
+<
 float2nr({expr})					*float2nr()*
 		Convert {expr} to a Number by omitting the part after the
 		decimal point.
@@ -7596,6 +7602,9 @@ prompt_getprompt({buf})					*prompt_getprompt()*
 		If the buffer doesn't exist or isn't a prompt buffer, an empty
 		string is returned.
 
+		Can also be used as a |method|: >
+			GetBuffer()->prompt_getprompt()
+
 prompt_setcallback({buf}, {expr})			*prompt_setcallback()*
 		Set prompt callback for buffer {buf} to {expr}.  When {expr}
 		is an empty string the callback is removed.  This has only
@@ -8376,7 +8385,9 @@ searchcount([{options}])					*searchcount()*
 						value. see |cursor()|, |getpos()
 						(default: cursor's position)
 
-
+		Can also be used as a |method|: >
+			GetSearchOpts()->searchcount()
+<
 searchdecl({name} [, {global} [, {thisblock}]])			*searchdecl()*
 		Search for the declaration of {name}.
 
@@ -9653,7 +9664,9 @@ strptime({format}, {timestring})				*strptime()*
 		  :echo strftime("%c", strptime("%Y%m%d%H%M%S", "19970427115355") + 3600)
 <		  Sun Apr 27 12:53:55 1997
 
-
+		Can also be used as a |method|: >
+			GetFormat()->strptime(timestring)
+<
 strridx({haystack}, {needle} [, {start}])			*strridx()*
 		The result is a Number, which gives the byte index in
 		{haystack} of the last occurrence of the String {needle}.
@@ -10526,6 +10539,9 @@ win_gettype([{nr}])					*win_gettype()*
 		popup window then 'buftype' is "terminal" and win_gettype()
 		returns "popup".
 
+		Can also be used as a |method|: >
+			GetWinid()->win_gettype()
+<
 win_gotoid({expr})					*win_gotoid()*
 		Go to window with ID {expr}.  This may also change the current
 		tabpage.
@@ -10581,6 +10597,9 @@ win_splitmove({nr}, {target} [, {options}])		*win_splitmove()*
 				present, the values of 'splitbelow' and
 				'splitright' are used.
 
+		Can also be used as a |method|: >
+			GetWinid()->win_splitmove(target)
+<
 							*winbufnr()*
 winbufnr({nr})	The result is a Number, which is the number of the buffer
 		associated with window {nr}.  {nr} can be the window number or
diff --git a/runtime/doc/testing.txt b/runtime/doc/testing.txt
index f0bda5aaf8..8ec66d26a4 100644
--- a/runtime/doc/testing.txt
+++ b/runtime/doc/testing.txt
@@ -157,6 +157,9 @@ assert_nobeep({cmd})					*assert_nobeep()*
 		produces a beep or visual bell.
 		Also see |assert_beeps()|.
 
+		Can also be used as a |method|: >
+			GetCmd()->assert_nobeep()
+<
 							*assert_notequal()*
 assert_notequal({expected}, {actual} [, {msg}])
 		The opposite of `assert_equal()`: add an error message to