Document HPM event 40 and external HPM events 41-50

[tejassinghbhati]

Summary

• document HPM event 40: stalls in the register-rename stage
• restore documentation for the external HPM events (41-50) exposed through the EXTERNAL_HPM_EVENT_NUM compile-time macro, including which configurations (4, 6, 10) expose each event

Why

top_drac.sv maps pmu_flags.stall_ir to HPM event 40, while events.md
previously stopped at event 39. In addition, the external PMU/HPM events
wired in via EXTERNAL_HPM_EVENT_NUM (L2/L1.5 access and miss, NOC flow and
stall events) were undocumented. Per review feedback on #7, the
configuration-validation RTL was dropped and only the documentation is kept.

Impact

Users configuring hardware performance counters can identify and select the
register-rename stall event and the external L2/L1.5/NOC events correctly.

Validation

• verified each documentation entry against the event mapping block in
  rtl/top_drac.sv (events 41-44 for any supported width, 45-46 for width 6,
  45-50 for width 10)
• documentation-only change; no HDL behavior changes

Related to #7