[clang-tidy objc-property-declaration] New option IncludeDefaultAcronyms

Summary:
The existing option objc-property-declaration.Acronyms
replaces the built-in set of acronyms.

While this behavior is OK for clients that don't want the default
behavior, many clients may just want to add their own custom acronyms
to the default list.

This revision introduces a new option,
objc-property-declaration.IncludeDefaultAcronyms, which controls
whether the acronyms in objc-property-declaration.Acronyms are
appended to the default list (the default behavior) or whether they
replace.

I also updated the documentation.

Test Plan: make -j12 check-clang-tools

Reviewers: Wizard, hokein, klimek

Reviewed By: hokein

Subscribers: Eugene.Zelenko, cfe-commits

Differential Revision: https://reviews.llvm.org/D42261

llvm-svn: 323130