Merging r370367:

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r370367 | kadircet | 2019-08-29 16:38:02 +0200 (Thu, 29 Aug 2019) | 13 lines

[clangd][NFC] Update background-index command line description

Summary:
We didn't change this in D64019 just in case we revert it back.
Deleting it now.

Reviewers: hokein, sammccall

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66943
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llvm-svn: 370370