Improve the description of the client config histograms.
Improve the description of the client config histograms.

Add some more details to the data reduction proxy client
config histograms.

BUG=

Review URL: https://codereview.chromium.org/1691683002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#374979}

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