I made userscript displaying the ::usage message of symbols when hovered in a code block:
It looks good for the vast majority of symbols but there are some limitations:
titleattribute used for tooltips doesn't allow html, causing some symbols for instance
Integrateto have "Subscript/Supscript/Subsuperscript" in the description:
I couldn't find good replacement for all the Private Use Area unicode symbols, for instance
I have v8 so symbols introduced in v9 will not get a tooltip.