Add document for query of search functions

[iamquang95]

Addressed the document part of #1266

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[codecov]

Codecov Report

Merging #1300 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1300   +/-   ##
=======================================
  Coverage   73.42%   73.42%           
=======================================
  Files          86       86           
  Lines        6047     6047           
=======================================
  Hits         4440     4440           
  Misses        837      837           
  Partials      770      770

Impacted Files	Coverage Δ
github/search.go	92.15% <ø> (ø)	⬆️

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[gmlewis]

nit: Could you please tweak this slightly so that it is easier to read? How about:

// If query includes multiple conditions, it MUST NOT include "+" as the condition separator.

[iamquang95]

Sure, i will add more detail (using " " instead) to the documentation. Thanks for your review.

[gmlewis]

Thank you, @iamquang95!
LGTM.
Merging.

[gmlewis]

Closes #1266.