Update mockReturnValue documentation

[jwbay]

Summary

• Un-deprecates mockReturnValue (fixes docs question - mockReturnValue deprecated? #3961) and describes its usage in terms of changing the
  return value of an existing mock. This was marked as deprecated for Jest 0.9.0 over a year ago, but I wasn't able to find any other mention of it. If mockReturnValue and mockReturnValueOnce are actually deprecated, I can switch the documentation to point to mockImplementation and mockImplementationOnce as replacements since they're closer analogues.
• Document mockReturnValueOnce with usage example instead of implementation

Test plan

[codecov-io]

Codecov Report

Merging #3993 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3993   +/-   ##
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  Coverage   60.26%   60.26%           
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  Files         196      196           
  Lines        6765     6765           
  Branches        6        6           
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  Hits         4077     4077           
  Misses       2685     2685           
  Partials        3        3

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