Create documentation for new REST endpoints

Changes for https://issues.couchbase.com/browse/MB-48480 introduce these endpoints: 

1) POST: /settings/alerts/limits

This end point supports the following parameters:

a) maxOverheadPerc b) maxDiskUsedPerc c) maxIndexerRamPerc

all percentages with valid ranges from 0 to 100 inclusive

2) GET: /settings/alerts/limits

This endpoint returns the 3 values noted above

These two endpoints are intended to be public and need to be documented.