closes #9764

The purpose of the {Nvim} hint was not well-defined, and its usage
inconsistent.  It's also unnecessary.

Nvim-Vim differences are centralized at:
    :help vim-differences
Removed things are centralized at:
    :help deprecated
Developer guidelines for documentation are listed at:
    :help dev-doc

- CMake features

Closes #8994

Problem:    Cannot act on edits in the command line.
Solution:   Add the CmdlineChanged autocommand event. (xtal8, closes vim/vim#2603,
            closes vim/vim#2524)

https://github.com/vim/vim/commit/153b704e20f9c269450a7d3ea8cafcf942579ab7

In Vim (and some vestigial parts of Nvim) E319 was a placeholder for
ex_ni commands, i.e. commands that are only available in certain builds
of Vim.  That is obviously counter to Nvim's goals: all Nvim commands
are available on all platforms and build types (the remaining ex_ni
commands are actually just missing providers).

We need an error id for "missing provider", so it makes sense to use
E319 for that purpose.

ref #9344
ref #3577

closes #1466
closes #1007
closes #1391

Problem:    No event triggered after updating diffs.
Solution:   Add the DiffUpdated event.

https://github.com/vim/vim/commit/e8fa05b5bc2d6d76bf5af50176a63655d00d1110

Problem:    Using an external diff program is slow and inflexible.
Solution:   Include the xdiff library. (Christian Brabandt)
            Use it by default.

https://github.com/vim/vim/commit/e828b7621cf9065a3582be0c4dd1e0e846e335bf

vim-patch:8.1.0360
vim-patch:8.1.0364
vim-patch:8.1.0366
vim-patch:8.1.0370
vim-patch:8.1.0377
vim-patch:8.1.0378
vim-patch:8.1.0381
vim-patch:8.1.0396
vim-patch:8.1.0432

- misc
- doc: `:help config`. closes #9329
- cleanup test/README.md