More tests inner parse + docs

[304NotModified]

added more unit tests. Moved innerlayout tests from whenTests to simpleLayoutParserTests

Side note:

I tried allowing balanced brackets (e.g. {} doesn't need an escape), but this cannot be done without being fully backwardscompatible

[codecov-io]

Current coverage is 70.65%

Merging #981 into master will decrease coverage by -0.03% as of b671b8a

@@            master    #981   diff @@
======================================
  Files          262     262       
  Stmts        14786   14792     +6
  Branches      1613    1614     +1
  Methods          0       0       
======================================
  Hit          10451   10451       
  Partial        401     401       
- Missed        3934    3940     +6

Review entire Coverage Diff as of b671b8a

---

Uncovered Suggestions

1. +0.09% via ...oredConsoleTarget.cs#232...244
2. +0.08% via ...c/NLog/LogFactory.cs#180...191
3. +0.07% via ...anceCounterTarget.cs#308...318
4. See 7 more...

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