Accessibility - SQL Indentation image alt text is the file name.

[jrchamp]

https://github.com/moodle/devdocs/blob/8a7b2f9c339fc6227f0016a8cd06b5c763dd2679/general/development/policies/codingstyle/sql.md?plain=1#L51

Produces an img element with alt="sql_indentation.png". Instead of providing the full description in the alternative text, I encourage providing a visible image caption so that everyone can benefit from a text description of the image's purpose.