fix #13218: avoid some irrelevant warnings for nim doc,rst2html,--app:lib, + other fixes

[timotheecour]

• fix nim doc generates lots of false positive LockLevel warnings #13218 avoid some irrelevant warnings for nim doc, nim rst2html, nim --app:llib
• alternative approach to Fix docgen snippet numbering #13507 (addresses a subset of the noise in ./koch docs; more could be done in future PR's); it's better to fix at the compiler level than patch every call to the compiler by inserting --warning:foo:off

[timotheecour]

@Araq PTAL
your exportc suggestion is good, will do in a separate PR

EDIT: failure unrelated, and I'm fixing it in Vladar4/nimgame2#41

https://dev.azure.com/nim-lang/Nim/_build/results?buildId=3274&view=logs&jobId=f6c2f757-7152-5b00-b42b-aaebf1a063dd&j=f6c2f757-7152-5b00-b42b-aaebf1a063dd&t=f0f21f3f-4495-55c9-9119-ef53cf103fc2

FAIL: https://github.com/Vladar4/nimgame2/ C
Test "https://github.com/Vladar4/nimgame2/" in category "nimble-packages"
Failure: reBuildFailed
package test failed
$ nim c nimgame2/nimgame.nim
Hint: used config file 'd:\a\1\s\config\nim.cfg' [Conf]
Hint: used config file 'd:\a\1\s\config\config.nims' [Conf]
Hint: system [Processing]
...
Hint: streams [Processing]
d:\a\1\s\pkgstemp\nimgame2\nimgame2\utils.nim(169, 54) Warning: use `csize_t` instead; csize is deprecated [Deprecated]
d:\a\1\s\pkgstemp\nimgame2\nimgame2\utils.nim(169, 31) Error: type mismatch: got <ptr RWops, ptr char, csize, int literal(1000)>
but expected one of: 
proc rwRead(context: ptr RWops; p: pointer; size: csize_t; maxnum: csize_t): csize_t
  first type mismatch at position: 3
  required type for size: csize_t
  but expression 'csize(1)' is of type: csize

expression: rwRead(src, addr(result[r]), csize(1), 1000)