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=.