[docs] Add more subsections

They are great!

doc_src/argparse.txt

@@ -15,6 +15,8 @@ Each option that is seen in the ARG list will result in a var name of the form `
 
 For example `_flag_h` and `_flag_help` if `-h` or `--help` is seen. The var will be set with local scope (i.e., as if the script had done `set -l _flag_X`). If the flag is a boolean (that is, does not have an associated value) the values are the short and long flags seen. If the option is not a boolean flag the values will be zero or more values corresponding to the values collected when the ARG list is processed. If the flag was not seen the flag var will not be set.
 
+\subsection argparse-options Options
+
 The following `argparse` options are available. They must appear before all OPTION_SPECs:
 
 - `-n` or `--name` is the command name to insert into any error messages. If you don't provide this value `argparse` will be used.

doc_src/bind.txt

@@ -54,6 +54,7 @@ The following parameters are available:
 
 - `-a` or `--all` See `--erase` and `--key-names`
 
+\subsection bind-functions Special input functions
 The following special input functions are available:
 
 - `accept-autosuggestion`, accept the current autosuggestion completely