improve search query syntax docs

Created by: sqs

• Removes usage of repogroup:sample, which is no longer needed on Sourcegraph.com
• Fixes many instances where the link text for an example differed from the linked query
• Fixes several instances where the example query was not a good example or did not have any current results on Sourcegraph.com
• Remove duplicate examples that only illustrated the alias when there was a non-alias example preceding it (eg if we have an example for repo:foo, another example for r:foo is not very helpful)
• Move timeout: and count: to the bottom