tests: string options in gen_opt_test.vim not fully sorted

tests: string options in gen_opt_test.vim not fully sorted

Problem:  tests: string options in gen_opt_test.vim aren't fully sorted.
Solution: Sort the string options alphabetically.  Also make description
          of 'maxsearchcount' start with lower-case for consistency with
          other options, update documentation for searchcount().

closes: #17720

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>