Adjust file finder width configuration (#20819)

Follow up to: https://github.com/zed-industries/zed/pull/18682

This PR tweaks the setting value, so it's clear we're referring to
`max-width`, meaning the width will change up to a specific value
depending on the available window size. Then, it also makes `Small` the
default value, which, in practice, makes the modal size the same as it
was before the original PR linked above.

Release Notes:

- N/A

---------

Co-authored-by: Kirill Bulatov <mail4score@gmail.com>

docs/src/configuring-zed.md

@@ -1418,11 +1418,11 @@ Or to set a `socks5` proxy:
 
 ## File Finder
 
-### Modal Width
+### Modal Max Width
 
-- Description: Width of the file finder modal. Can take one of a few values: `small`, `medium`, `large`, `xlarge`, and `full`.
-- Setting: `modal_width`
-- Default: `medium`
+- Description: Max-width of the file finder modal. It can take one of these values: `small`, `medium`, `large`, `xlarge`, and `full`.
+- Setting: `max_modal_width`
+- Default: `small`
 
 ## Preferred Line Length