doc: normalize C code block info strings

Prior to this commit, C fenced code blocks in Markdown files had inconsistent info strings. This has been corrected to standardize on the lowercase variant, which matches the others, is the typical file extension for C files, and is the CSS class used by highlight.js despite representing a minority of appearances in the doc/api/ dir.

Stats:

'C' => 181, 'c' => 3,

Refs: highlightjs/highlight.js#2577

Checklist

• documentation is changed or added
• commit message follows commit guidelines