Put a tooltip with ::usage on builtin symbols on mathematica.stackexchange.com and Wolfram Community
The script does no attempt to parse code but relies on the fact that both mma.SE (Thanks to halirutans work on lang-mma.js) and WC tag builtin symbols with a special class, making them easy to detect.
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Sub and supescripts are only partially supported by unicode, so some descriptions have the full-form "Subscript" and "Subsuperscript" (Same goes for under and overscript that is completely missing)
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Mathematica uses a lot of symbols in the unicode PUA. I've mapped the ones I could find similar looking characters for, others are left as-is (See Conjugate for instance)
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The ::usage was taken from v8 so any newer symbols are not included