apt-*: add more info links; apt-file: add regex search example (#5710)

pages/linux/apt-file.md

@@ -1,6 +1,7 @@
 # apt-file
 
 > Search for files in apt packages, including ones not yet installed.
+> More information: <https://manpages.debian.org/latest/apt-file/apt-file.1.html>.
 
 - Update the metadata database:
 
@@ -8,8 +9,12 @@
 
 - Search for packages that contain the specified file or path:
 
-`apt-file search {{part/of/filename}}`
+`apt-file {{search|find}} {{part/of/filename}}`
 
 - List the contents of a specific package:
 
-`apt-file list {{package_name}}`
+`apt-file {{show|list}} {{package_name}}`
+
+- Search for packages that match the regular expresssion given in `pattern`:
+
+`apt-file {{search|find}} --regexp {{pattern}}`