gh-133286: add explanation about seq for pathlib Pattern Language (GH-133340)

(cherry picked from commit ac8df4b)

Co-authored-by: alexey semenyuk <alexsemenyuk88@gmail.com>

Author: alex-semenyuk

Doc/library/pathlib.rst

@@ -1661,9 +1661,12 @@ The following wildcards are supported in patterns for
 ``?``
   Matches one non-separator character.
 ``[seq]``
-  Matches one character in *seq*.
+  Matches one character in *seq*, where *seq* is a sequence of characters.
+  Range expressions are supported; for example, ``[a-z]`` matches any lowercase ASCII letter.
+  Multiple ranges can be combined: ``[a-zA-Z0-9_]`` matches any ASCII letter, digit, or underscore.
+
 ``[!seq]``
-  Matches one character not in *seq*.
+  Matches one character not in *seq*, where *seq* follows the same rules as above.
 
 For a literal match, wrap the meta-characters in brackets.
 For example, ``"[?]"`` matches the character ``"?"``.