Update HTTP API documentation re: bind params

[timhallinflux]

URL for relevant page?

https://docs.influxdata.com/influxdb/v1.7/tools/api/#bind-parameters

What products and version are you using?

Documentation describes that bind-parameters is only for the where clause. This changes with 1.8. See details here: influxdata/influxql#34

[jsternberg]

To clarify, bind parameters have always worked everywhere in the query. In 1.8, bind parameters will now support more parts of the AST they can bind too. The above change makes it so you can bind identifiers, regexes, and durations.

The existing bind parameters have always worked everywhere in the query. It has been possible since the first version of bind parameters to do LIMIT $n because n is an integer literal.

[pierwill]

Closed via #2710.