Accordly with the [nmap documentation][1] in "Example 9.2. script help", the nmap's parameter that displays help about the script is:

    nmap --script-help <filename.nse>

Namely:

    nmap --script-help http-sql-injection.nse

If you look at [NSE documentation][2] written by own Nmap's creator:

> **3.1. description Field**
> 
> The description field describes what a script is testing for and any importantnotes the user should beaware of.
> Depending on script complexity, the description may vary from a few
> sentences to a few paragraphs. The first paragraph should be a brief
> synopsis of the script function suitable for stand-alone presentation
> to the user. Further paragraphs may provide much more script detail.

Fyodor explain that the "description" field is used to describe everything about the complexity that the script itself might have. So, go ahead and look deeper into how the script was made:

     $ vi /usr/share/nmap/scripts/http-sql-injection.nse
...

     11 description = [[
     12 Spiders an HTTP server looking for URLs containing queries vulnerable to an SQL
     13 injection attack. It also extracts forms from found websites and tries to identify
     14 fields that are vulnerable.
     15 
     16 The script spiders an HTTP server looking for URLs containing queries. It then
     17 proceeds to combine crafted SQL commands with susceptible URLs in order to
     18 obtain errors. The errors are analysed to see if the URL is vulnerable to
     19 attack. This uses the most basic form of SQL injection but anything more
     20 complicated is better suited to a standalone tool.
     21 
     22 We may not have access to the target web server's true hostname, which can prevent access to
     23 virtually hosted sites.
     24 ]]

This explain why the `http-sql-injection.nse` script do not shown "usage options" whether that's what you expected to see.


----------
The NSE scripts has a tradition to include "usage options" inside a comments. Such thing that was documented on the section "**8.1 The Head**":

> Next comes NSEDoc information. This script is missing the common
> @usage and @args tags since it is so simple, but it does have an
> NSEDoc @output tag:
> 
>     ---
>     --@output
>     -- 21/tcp   open     ftp       ProFTPD 1.3.1
>     -- |_ auth-owners: nobody
>     -- 22/tcp   open     ssh       OpenSSH 4.3p2 Debian 9etch2 (protocol 2.0)
>     -- |_ auth-owners: root
>     -- 25/tcp   open     smtp      Postfix smtpd
>     -- |_ auth-owners: postfix
>     -- 80/tcp   open     http      Apache httpd 2.0.61 ((Unix) PHP/4.4.7 ...)
>     -- |_ auth-owners: dhapache
>     -- 113/tcp  open     auth?
>     -- |_ auth-owners: nobody
>     -- 587/tcp  open     submission Postfix smtpd
>     -- |_ auth-owners: postfix
>     -- 5666/tcp open     unknown
>     -- |_ auth-owners: root

Since the documentation does not explicitly show the use of "@usage", look yourself in `/usr/share/nmap/scripts/`:

    $ grep -iR -A5 "@usage" /usr/share/nmap/scripts/  
    /usr/share/nmap/scripts/ajp-brute.nse:-- @usage
    /usr/share/nmap/scripts/ajp-brute.nse--- nmap -p 8009 <ip> --script ajp-brute
    /usr/share/nmap/scripts/ajp-brute.nse---
    /usr/share/nmap/scripts/ajp-brute.nse--- @output
    /usr/share/nmap/scripts/ajp-brute.nse--- PORT     STATE SERVICE
    /usr/share/nmap/scripts/ajp-brute.nse--- 8009/tcp open  ajp13
    ...



  [1]: https://nmap.org/book/nse-usage.html
  [2]: https://media.blackhat.com/bh-us-10/whitepapers/Vaskovitch/BlackHat-USA-2010-Fyodor-Fifield-NMAP-Scripting-Engine-wp.pdf