Clarify the logical counter index in the snapshot area.

The counter start/stop function clearly states that snapshot shared memory
area is only valid for the counters being stopped/started in the respective
calls. That means the logical counter index modified must be relative to the
counter base provided in the start/stop call. Add more clarification to avoid
any ambiguity.

Signed-off-by: Atish Patra <atishp@rivosinc.com>

src/ext-pmu.adoc

@@ -538,21 +538,28 @@ implement it.
 |===
 | Name                    | Offset   | Size | Description
 | counter_overflow_bitmap | 0x0000   | 8    | A bitmap of all logical overflown
-                                              counters. This is valid only if
-                                              the `Sscofpmf` ISA extension is
-                                              available. Otherwise, it must be
-                                              zero.
+                                              counters relative to the
+					      `counter_idx_base`. This is valid
+					      only if the `Sscofpmf` ISA
+					      extension is available. Otherwise,
+					      it must be zero.
 | counter_values          | 0x0008   | 512  | An array of 64-bit logical
                                               counters where each index
                                               represents the value of each
                                               logical counter associated with
-                                              hardware/firmware.
+                                              hardware/firmware relative to the
+					      `counter_idx_base`.
 | Reserved                | 0x0208   | 3576 | Reserved for future use
 |===
 
 Any future revisions to this structure should be made in a backward compatible
 manner and will be associated with an SBI version.
 
+The logical counter indicies in the `counter_overflow_bitmap` and `counter_values`
+array are relative w.r.t to `counter_idx_base` argument present in the
+`sbi_pmu_counter_stop` and `sbi_pmu_counter_start` functions. This allows the
+users to use snapshot feature for more than XLEN counters if required.
+
 This function should be invoked only once per hart at boot time. Once
 configured, the SBI implementation has read/write access to the shared memory
 when `sbi_pmu_counter_stop` is invoked with the