Improve documentation for `esc`

esc(e)

  Only valid in the context of an Expr returned from a macro. Prevents the macro hygiene pass from turning embedded variables into gensym variables. See the Macros section of the Metaprogramming chapter of the manual for more details and examples.

I'm not sure what "valid" really means here, since

julia> esc(:(x, ))
:($(Expr(:escape, :((x,)))))

It could mean "you can't eval the resulting expression":

julia> eval(esc(:(x, )))
ERROR: syntax: invalid syntax (escape (call (core tuple) (outerref x)))