i#4313: Clarify docs regarding first, first_nonlabel, and last instr functions (#4277)
i#4313: Clarify docs regarding first, first_nonlabel, and last instr functions (#4277)

Adds documentation to clarify that users should not use drmgr variants to check for first, first_nonlabel and last instructions.

Issue: #4134
AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio