Small doc improvements for std/with

[konsumlamm]

Some notes:

• The since pragma can't be used here (see template/macro pragmas don't work in templates (eg: byaddr, since etc) #15920), at least it has no effect on the generated documentation.
• Is there any particular reason why with is not part of the sugar module? If not, should this perhaps be reexported by sugar in the future?
• Currently, the with macro evaluates arg multiple times, should this be changed to only evaluate it once? If not, what are the reasons for this (should be documented imo)?

[timotheecour]

add / integrate an example showing _ and add explain we can use _ in the doc comment (allows cases where we don't replace the "first argument")

  var x = 1
  var y = 100
  with x:
    +=2
    *=10
    y+=_
  assert (x, y) == (30, 130)

[timotheecour]

Is there any particular reason why with is not part of the sugar module? If not, should this perhaps be reexported by sugar in the future?

IMO we should, in future PR. I'm in favor of splitting large modules (times, os, sugar etc) into smaller ones + re-exporting those in the original large module, which allows granular access when needed (cyclic dependencies etc) or easy access to all symbols by default.

Currently, the with macro evaluates arg multiple times, should this be changed to only evaluate it once? If not, what are the reasons for this (should be documented imo)?

trickier than it seems, but it's possible, using similar implementation as (or reusing) #13750, but that would deserve its own PR. evalOnce takes care of the diffculty of dealing both with lvalues (no copying) and rvalues. (see also #13245 chainEval which IIRC avoided multiple evaluations but did a copy even for lvalues)