d: Update documentation of new D language options.

Adds documentation for the following:

    - New switch that controls what code is generated on a contract
      failure (throw or abort).
    - New switch that controls mangling of D types in `extern(C++)`
      code, as well as setting the compile-time value of
      `__traits(getTargetInfo "cppStd")`
    - New switches that generate C++ headers from D source files.
    - New switch to save expanded mixins to a file.
    - New switches that now distinguish between D language changes that
      are either (a) an experimental feature or an upcoming breaking
      change, (b) a warning or help on an upcoming change, or (c) revert
      of a change for users who don't want to deal with the breaking
      change for now.

gcc/d/ChangeLog:

	* gdc.texi (Runtime Options): Document -fcheckaction=, -fextern-std=,
	-fpreview=, -frevert=.
	(Code Generation): Document -fdump-c++-spec=, -fdump-c++-spec-verbose,
	-fsave-mixins=.
	(Warnings): Update list of supported -ftransitions=.

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