docs: Actually document commandline --input

This is a useful option and has been a thing for years.

doc_src/cmds/commandline.rst

@@ -74,6 +74,9 @@ The following options change what part of the commandline is printed or updated:
 **--search-field**
     Use the pager search field instead of the command line. Returns false if the search field is not shown.
 
+**--input=INPUT**
+    Operate on this string instead of the commandline. Useful for using options like **--tokens-expanded**.
+
 The following options change the way ``commandline`` prints the current commandline buffer:
 
 **-c** or **--cut-at-cursor**