Shell: Document the use of environment/local variables

Author: https://github.com/alimpfard
Commit: https://github.com/SerenityOS/serenity/commit/68189f80ccb
Pull-request: https://github.com/SerenityOS/serenity/pull/3199

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

@@ -38,3 +38,7 @@ Shell -c 'rm foo*'
 # Execute the contents of a file with some arguments
 Shell foo a b c
 ```
+
+## See also
+
+* [`Shell-vars`(7)](../man7/Shell-vars.md) For details on local and environment variables used by the shell

Base/usr/share/man/man7/Shell-vars.md

@@ -0,0 +1,37 @@
+## Name
+
+Shell Variables - Special local and environment variables used by the Shell
+
+## Description
+
+The Shell uses various variables to allow for customisations of certain behavioural or visual things.
+Such variables can be changed or set by the user to tweak how the shell presents things.
+
+## Behavioural
+
+1. Output interpretations
+
+`IFS` (local)
+
+The value of this variable is used to join lists or split strings into lists, its default value is a newline (`\\n`).
+
+
+## Visual
+
+2. Prompting
+
+`PROMPT` (environment)
+
+The value of this variable is used to generate a prompt, the following escape sequences can be used literally inside the value, and they would expand to their respective values:
+- `\\u` : the current username
+- `\\h` : the current hostname
+- `\\w` : a collapsed path (relative to home) to the current directory
+- `\\p` : the string '$' (or '#' if the user is 'root')
+
+Any other escaped character shall be ignored.
+
+
+`PROMPT_EOL_MARK` (environment)
+
+The value of this variable is used to denote the ends of partial lines (lines with no newline), its default value is '%'.
+