Press CTRL-F (find/replace), and you'll note that even in Excel 2013 CTRL-A (select all) doesn't work in either the find or replace boxes.
It has been this way since at least Office 95.
Imagine you'll be able access this through the online help:
https://support.office.com/en-us/article/Excel-functions-by-...
The offline help is fine for Excel functions. But where I work, 90% of the functions used are coming from addins, and even though they also have a documentation somewhere, it can't be reached with F1.
Documentation is necessary but hints should be inline/contextual, like intellisense in visual studio. I don't want to have to leave what I am doing just to figure out which is the next argument in a function.
Excel could use a powerful autocomplete like intellisense that gives you basic documentation inside the auto-complete.
But Office is the "sleeping beauty" of Microsoft. A bit of an old and hairy beauty though.