Address aspnet/razor#236, recreate XML files when xml-docs-test target is run directly
- must still `build initialize` between `git clean` and `build xml-docs-test`

nit: add description of xml-docs-test target
Correct StyleCop violations
- StyleCop working again (handles C# 6.0 additions) though only locally for me
- disable new PrefixCallsCorrectly rule
- correct remaining violations
 - lots of long lines and out-of-order or misnamed `static readonly` fields

nit: remove new trailing whitespace (was paranoid about adding it w/ fixes)