Add documentation for completions command (#2126)

COMMANDS.md

@@ -21,6 +21,7 @@ The most common subcommands are:
 * [`pyenv prefix`](#pyenv-prefix)
 * [`pyenv hooks`](#pyenv-hooks)
 * [`pyenv shims`](#pyenv-shims)
+* [`pyenv completions`](#pyenv-completions)
 
 ## `pyenv commands`
 
@@ -345,3 +346,9 @@ List existing pyenv shims.
     /home/user/.pyenv/shims/python3.9-gdb.py
     /home/user/.pyenv/shims/python3-config
     /home/user/.pyenv/shims/python-config
+
+## `pyenv completions`
+
+Lists available completions for a given pyenv command.
+
+    Usage: pyenv completions <command> [arg1 arg2...]