pgrep: Add -U option to filter matches by UID or login name

Author: https://github.com/tcl3
Commit: 821bf5e071
Pull-request: https://github.com/SerenityOS/serenity/pull/19155

Base/usr/share/man/man1/pgrep.md

@@ -5,7 +5,7 @@ pgrep - look up processes based on name
 ## Synopsis
 
 ```sh
-$ pgrep [--count] [-d delimiter] [--ignore-case] [--list-name] [--invert-match] <process-name>
+$ pgrep [--count] [-d delimiter] [--ignore-case] [--list-name] [--uid uid-list] [--invert-match] <process-name>
 ```
 
 ## Options
@@ -14,6 +14,8 @@ $ pgrep [--count] [-d delimiter] [--ignore-case] [--list-name] [--invert-match]
 * `-d`, `--delimiter`: Set the string used to delimit multiple pids
 * `-i`, `--ignore-case`: Make matches case-insensitive
 * `-l`, `--list-name`: List the process name in addition to its pid
+* `-U uid-list`, `--uid uid-list`: Select only processes whose UID is in the given comma-separated list. Login name or numerical user ID may be used
+
 * `-v`, `--invert-match`: Select non-matching lines
 
 ## Arguments