[checks/property-decls] Fix comment in clang-tidy/objc/PropertyDeclarationCheck.cpp ✍️

Summary: The comment incorrectly claims that the listed acronyms are all extracted from the linked Apple documentation.

Reviewers: Wizard, benhamilton

Reviewed By: Wizard, benhamilton

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D46922

Contributed by @stephanemoore.

llvm-svn: 334238