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
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documentation is changed or added -
commit message follows commit guidelines