documentation: Update assert_{false,true}() and empty() documentation

Also removes a note regarding the performance of `empty(long_list)` vs
`len(long_list) == 0` because this has nothing to do with the actual state:
first checks that list is not NULL and pointer to its first element is also not
NULL, second gets length by comparing list with NULL and falls back to
`tv->vval.v_list->lv_len` if not. `len(long_list)` *may* still be *slightly*
slower, but the slow down has nothing to do with the length of the list, is
hardly noticeable and depends on how good compiler is at inlining and what
exactly have author of the plugin written (I mean `len(long_list) == 0` vs
`empty(long_list)` vs `!len(long_list)`).

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