documentation of {.noRewrite.} pragma is missing

[mratsim]

While looking into term-rewriting macros recursion, I realised that the one true fix to #9288 #8376 and #2901 mentioned in the doc PR #9305 was introduced in 2015, the forgotten {.noRewrite.} pragma 4674b3e.

It's completely undocumented though and I'm not even sure @Araq was aware of it:

Introduce {.noRewrite.} expr pragma disabling TR

Term rewriting macros/templates are currently greedy and they will rewrite as
long as there is a match. So there was no way to ensure some rewrite happens
only once, eg. when rewriting term to same term plus extra content.

With new macro we can actually prevent further rewriting on marked expr or
stmts, eg. with given example echo(...) will be rewritten just once:

template pwnEcho{echo(x)}(x: expr) =
  {.noRewrite.}: echo("pwned!")

echo "ab"

cc @andreaferretti @kaushalmodi @reactormonk

[Araq]

I'm aware but I don't think it's a good idea to document it, this solution doesn't feel right.