Search docs do not include all operators

Created by: sqs

@lhorie reported that our search help page (https://about.sourcegraph.com/docs/search/query-syntax) needs improvement at https://news.ycombinator.com/item?id=18118409.

TODOs:

• add count/max - done
• add more diff search examples
• add more examples overall (@francisschmaltz's new search page designs will help here too)
• add info about what features are missing and when we plan to add them (e.g., multi-line search)