Add docs for new "mainthread" command option in NUnitlite 4.6+

[stevenaw]

Relates to nunit/nunit#4240

The framework has supported a "RunOnMainThread" setting for a while but it hasn't been readily exposed in our docs, leading to many to continue to request it as a feature. That PR adds support to pass it on the command line to nunitlite, which should be documented as coming up in the 4.6 release.

• Write the docs
• Remove requires:docs label from the originating GH issue