[clangd] Mention indexing in docs.

[kadircet]

• Change ordering of sentences

[sammccall]

Sorry about delaying this.

docs/clangd.rst

156

I think we should be a little more specific here: which will incrementally build an index of the projects that you work on

160

nit: clangd-indexer rather than clangd_indexer, I think.
Maybe slightly more explicit: *Note that clangd-indexer isn't included alongside clangd in the standard clang-tools package. You will likely have to build from source to use this option*

[kadircet]

Address comments

docs/clangd.rst

 from :program:`VSCode`.
 
 Emacs Integration
 -----------------
 
 :program:`Emacs` provides `lsp-mode <github.com/emacs-lsp/lsp-mode>`_ and
 `Eglot <https://github.com/joaotavora/eglot>`_ plugins for LSP integration.
 
+Project-wide Index
+==================
+
+By default :program:`Clangd` only has a view on symbols coming from files you
+are currently editing. You can extend this view to whole project by providing a
+project-wide index to :program:`Clangd`.
+
+There are two ways you can generate a project-wide index for clangd:
+
+- Passing experimental `-background-index` commandline argument, which will
+  incrementally build an index of projects that you work on and make use of that
+  in clangd automatically.
+- Generate an index file using `clangd-indexer
+  <https://github.com/llvm-mirror/clang-tools-extra/blob/master/clangd/indexer/IndexerMain.cpp>`_
+  Afterwards you can pass generated index file to clangd using
+  `-index-file=/path/to/index_file`.  *Note that clangd-indexer isn't included
+  alongside clangd in the standard clang-tools package. You will likely have to
+  build from source to use this option*
+
 Getting Involved
 ==================
 
 A good place for interested contributors is the `Clangd developer mailing list
 <https://lists.llvm.org/mailman/listinfo/clangd-dev>`_. For discussions with the
 broader community on topics not only related to Clangd, use
 `Clang developer mailing list
 <https://lists.llvm.org/mailman/listinfo/cfe-dev>`_.
 If you're also interested in contributing patches to :program:`Clangd`, take a
 look at the `LLVM Developer Policy
 <https://llvm.org/docs/DeveloperPolicy.html>`_ and `Code Reviews
 <https://llvm.org/docs/Phabricator.html>`_ page. Contributions of new features
 to the `Language Server Protocol
 <https://github.com/Microsoft/language-server-protocol>`_ itself would also be
 very useful, so that :program:`Clangd` can eventually implement them in a
 conforming way.