[clangd] Describe non-handling of most IWYU pragmas. NFC

clang-tools-extra/clangd/index/CanonicalIncludes.h

 };
 
 /// Returns a CommentHandler that parses pragma comment on include files to
 /// determine when we should include a different header from the header that
 /// directly defines a symbol. Mappinps are registered with \p Includes.
 ///
 /// Currently it only supports IWYU private pragma:
 /// https://github.com/include-what-you-use/include-what-you-use/blob/master/docs/IWYUPragmas.md#iwyu-pragma-private
+///
+/// We ignore other pragmas:
+/// - keep: this is common but irrelevant: we do not currently remove includes
+/// - export: this is common and potentially interesting, there are three cases:
+///    * Points to a public header (common): we can suppress include2 if you
+///      already have include1. Only marginally useful.
+///    * Points to a private header annotated with `private` (somewhat commmon):
+///      Not incrementally useful as we support private.
+///    * Points to a private header without pragmas (rare). This is a reversed
+///      private pragma, and is valuable but too rare to be worthwhile.
+/// - no_include: this is about as common as private, but only affects the
+///   current file, so the value is smaller. We could add support.
+/// - friend: this is less common than private, has implementation difficulties,
+///   and affects behavior in a limited scope.
+/// - associated: extremely rare
 std::unique_ptr<CommentHandler>
 collectIWYUHeaderMaps(CanonicalIncludes *Includes);
 
 } // namespace clangd
 } // namespace clang
 
 #endif // LLVM_CLANG_TOOLS_EXTRA_CLANGD_INDEX_HEADERMAPCOLLECTOR_H