[clangd] Clarify and hide -index flag.

[ioeric]

Summary

The wording implies global index support, which is confusing.
As most users shouldn't care about this flag, also make it hidden to avoid
further confusion.

[ilya-biryukov]

+1 to making this hidden. Users (most of them) shouldn't care about it.
Is there any motivation on why we someone would want to disable it currently? Maybe we want to deprecate it and remove it completely at some point?
I could only think of being as close as possible to sema completions, but that doesn't seem like something that gives better UX (folks from Apple might be interested in that, but not sure).

LGTM from my side.

clangd/tool/ClangdMain.cpp

135 ↗

(On Diff #165061)

Maybe replace static/global with simply "Global" or "Project-wide".
I think it's good that we don't mention "dynamic" and "static" index in the docs too much, users shouldn't really care about this terminology.

[ioeric]

• Remove static.

clang-tools-extra/trunk/clangd/tool/ClangdMain.cpp

 static llvm::cl::opt<Path> InputMirrorFile(
     "input-mirror-file",
     llvm::cl::desc(
         "Mirror all LSP input to the specified file. Useful for debugging."),
     llvm::cl::init(""), llvm::cl::Hidden);
 
 static llvm::cl::opt<bool> EnableIndex(
     "index",
-    llvm::cl::desc("Enable index-based features such as global code completion "
-                   "and searching for symbols. "
-                   "Clang uses an index built from symbols in opened files"),
-    llvm::cl::init(true));
+    llvm::cl::desc(
+        "Enable index-based features. By default, clangd maintains an index "
+        "built from symbols in opened files. Global index support needs to "
+        "enabled separatedly."),
+    llvm::cl::init(true), llvm::cl::Hidden);
 
 static llvm::cl::opt<bool>
     ShowOrigins("debug-origin",
                 llvm::cl::desc("Show origins of completion items"),
                 llvm::cl::init(clangd::CodeCompleteOptions().ShowOrigins),
                 llvm::cl::Hidden);
 
 static llvm::cl::opt<bool> HeaderInsertionDecorators(