add more details to some flags' doc

Summary: I think it should help. Wait for insights from folly experts

Reviewed By: Gownta

Differential Revision: D67546034

fbshipit-source-id: 72c83f43d2513440262871c47c86a892c91b563e

Author: Zhijian Jiang

third-party/folly/src/folly/Benchmark.cpp

@@ -46,7 +46,10 @@ FOLLY_GFLAGS_DEFINE_bool(benchmark, false, "Run benchmarks.");
 
 FOLLY_GFLAGS_DEFINE_bool(json, false, "Output in JSON format.");
 
-FOLLY_GFLAGS_DEFINE_bool(bm_estimate_time, false, "Estimate running time");
+FOLLY_GFLAGS_DEFINE_bool(
+    bm_estimate_time,
+    false,
+    "Estimate running time by returning the geometric mean of latency values between p25 and p75.");
 
 #if FOLLY_PERF_IS_SUPPORTED
 FOLLY_GFLAGS_DEFINE_string(