Accordly with the nmap documentation 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 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.