[docs] AppleDocMerger make the missing type count configurable

If a specific number of types are missing AppleDocMerger throws. This was hard-coded before and now this is configurable via the command-line which is useful when building against different documentation sets.
[docs] Enable missing type count configuration from the docfixer command-line
[docs] docfixer: Make the compiled doc bundle configurable from the command-line

Additionally, this patch sets the default archive for MonoMac and MonoTouch eliminating the need to compile in docfixer.mm.cs or docvixer.mt.cs. The invoker is also notified if the doc bundle does not exist.
[docs] document-generated-code: Make namespace prefix and Apple doc location command-line configurable

This removes references to MonoTouch and makes the namespace configurable from the command-line. Additionally, the Apple docs location can also be specified when a merge should be performed. This eliminates the "appledocs" parameter in favor of "apple-doc-dir" for consistency with docfixer.