i#2350 rseq: Update rseq limitation docs (#4027)
i#2350 rseq: Update rseq limitation docs (#4027)

Updates a now-stale detail in the rseq limitation docs: we no longer
try to analyze read-write sequences for restoring state for the second
rseq execution.  We do still limit our checkpointing to
general-purpose registers.

Issue: #2350
AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio