Outdated API in docs for macros

[Blaisorblade]

1. docs/docs/reference/metaprogramming/macros.md uses sometimes toExpr for Expr():
   4b732ce#r36100940

2. macros.md doesn't say where to get QuoteContext. Today two people asked questions on this point on Gitter.

[nicolasstucki]

The QuoteContex is given within a top level $ in an online method. Or if using staging, the run or withQuoteContext provides it.

[machisuji]

There were some other issues I stumbled upon. I created another PR to (#7611) point those out. Not sure if that was the right thing to do. Please let me how else to do this in the future if it wasn't.