Improve documentation for `HEAP_PROF_SAMPLE_STRING` and `HEAP_PROF_SAMPLE_COST_CENTRE`
This MR improves the documentation for the HEAP_PROF_SAMPLE_STRING and HEAP_PROF_SAMPLE_COST_CENTRE events.
It adds a more detailed description of when each is emitted, and adds a more complete description of the contents of the sample label field of the HEAP_PROF_SAMPLE_STRING event, broken down by heap profile type.
This MR makes no changes to GHC, only to the GHC users guide.