worktree: improve find_worktree() documentation

worktree: improve find_worktree() documentation

Do a better job of explaining that find_worktree()'s main purpose is to
locate a worktree based upon input from a user which may be some sort of
shorthand for identifying a worktree rather than an actual path. For
instance, one shorthand a user can use to identify a worktree is by
unique path suffix (i.e. given worktrees at paths "foo/bar" and
"foo/baz", the latter can be identified simply as "baz"). The actual
heuristics find_worktree() uses to select a worktree may be expanded in
the future (for instance, one day it may allow worktree selection by
<id> of the .git/worktrees/<id>/ administrative directory), thus the
documentation does not provide a precise description of how matching is
performed, instead leaving it open-ended to allow for future
enhancement.

While at it, drop mention of the non-NULL requirement of `prefix` since
NULL has long been allowed. For instance, prefix_filename() has
explicitly allowed NULL since 116fb64e43 (prefix_filename: drop length
parameter, 2017-03-20), and find_worktree() itself since e4da43b1f0
(prefix_filename: return newly allocated string, 2017-03-20).

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>