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No change to logic, only docs. This updates java_cpp_strings docs:

 * The first paragraph incorrectly stated this uses comments to
   determine the Java class name. This corrects that statement.
 * Merges the two GN steps into one, explains it's a best practice to
   add this to an android_library (or create a new one if there is
   none), and recommends private visibility for :java_switches_srcjar.

Test: Upload to gerrit > open file > click "gitiles"
Change-Id: I4ae45da15f7db02a9d4c101de1a23c40f154b464
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2369652
Commit-Queue: Bo <boliu@chromium.org>
Auto-Submit: Nate Fischer <ntfschr@chromium.org>
Reviewed-by: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800838}