misc documentation (not docgen)

[timotheecour]

• mention in docs that mixin is also a 2nd class annotation, see D20200419T032540

[timotheecour]

• document that one difference bw inc and += is that:
  var a: uint8
  var b = 1
  inc(a, b) # wokrs
  a+=b # CT error

[timotheecour]

document this: https://github.com/nim-lang/Nim/pull/13451/files#r385118474
when to use assert/doAssert vs throwing catchables for API's

[timotheecour]

how to introduce breaking changes: see nim-lang#14048 (comment)

[timotheecour]

this seems wrong at least for arc, and for future language spec

For let symbols a copy is not always necessary:

let s = varA    # may only copy a pointer if it safe to do so

[timotheecour]

manual should specify that =destroy (+ a few others to specify) is special and doesn't need to be public (cf: type bound operators)

[timotheecour]

add doc comment in io.open to recommend using the overload of open that raises, refs https://github.com/nim-lang/Nim/pull/16320/files#r542009301

[timotheecour]

extend documentation for importcpp, eg how to wrap const char*

refs: nim-lang#3720 (comment)

[timotheecour]

• removePrefix + friends don't document behavior when prefix isn't found in string

[timotheecour]

• {.explain.} seems useful but not documented in manual; see usage exmple here:
  $nim_prs_D/tests/concepts/texplain.nim

[timotheecour]

• document this importcpp pattern:

nim-lang#4441 (comment)

You can also import constants as nullary functions instead:

proc foo(): cint {.importcpp: "FOO@".}

I'm not sure it really needs documentation. It's a clever use of the
importcpp pattern language that (I confess) I handn't thought of.
{.importcpp: "...".} can be used with the C backend. Perhaps that should
be documented. Using it this way does worry me a bit, though.

[timotheecour]

• document that rtl procs can't be inline, refs add strbasics.strip nim-lang/Nim#16280 (comment)