linux/u*: add links (#6190)

pages/linux/ul.md

@@ -2,6 +2,7 @@
 
 > Performs the underlining of a text.
 > Each character in a given string must be underlined separately.
+> More information: <https://manned.org/ul>.
 
 - Display the contents of the file with underlines where applicable:
 

pages/linux/unix2dos.md

@@ -2,6 +2,7 @@
 
 > Change Unix-style line endings to DOS-style.
 > Replaces CR with CRLF.
+> More information: <https://waterlan.home.xs4all.nl/dos2unix.html>.
 
 - Change the line endings of a file:
 

pages/linux/unix2mac.md

@@ -2,6 +2,7 @@
 
 > Change Unix-style line endings to macOS-style.
 > Replaces CR with LF.
+> More information: <https://waterlan.home.xs4all.nl/dos2unix.html>.
 
 - Change the line endings of a file:
 

pages/linux/unset.md

@@ -1,6 +1,7 @@
 # unset
 
 > Remove shell variables or functions.
+> More information: <https://manned.org/unset>.
 
 - Remove the variable `foo`, or if the variable doesn't exist, remove the function `foo`:
 

pages/linux/update-alternatives.md

@@ -1,6 +1,7 @@
 # update-alternatives
 
 > A convenient tool for maintaining symbolic links to determine default commands.
+> More information: <https://manned.org/update-alternatives>.
 
 - Add a symbolic link:
 

pages/linux/update-rc.d.md

@@ -2,6 +2,7 @@
 
 > Install and remove services which are System-V style init script links.
 > Init scripts are in the `/etc/init.d/`.
+> More information: <https://manned.org/update-rc.d>.
 
 - Install a service:
 

pages/linux/updatedb.md

@@ -2,6 +2,7 @@
 
 > Create or update the database used by `locate`.
 > It is usually run daily by cron.
+> More information: <https://manned.org/updatedb>.
 
 - Refresh database content: