Describe the default benchmark duration as an estimate

README.md

@@ -33,8 +33,9 @@ hyperfine 'sleep 0.3'
 ```
 
 Hyperfine will automatically determine the number of runs to perform for each command. By default,
-it will perform *at least* 10 benchmarking runs and measure for at least 3 seconds. To change this,
-you can use the `-r`/`--runs` option:
+it will perform *at least* 10 benchmarking runs and estimate a run count targeting roughly
+3 seconds, including shell overhead and preparation and conclusion commands. To set an exact
+number of runs, you can use the `-r`/`--runs` option:
 ```sh
 hyperfine --runs 5 'sleep 0.3'
 ```
@@ -89,7 +90,7 @@ hyperfine -L compiler gcc,clang '{compiler} -O2 main.cpp'
 
 ### Intermediate shell
 
-By default, commands are executed using `sh` on Unix (resolved through `PATH`) or `cmd.exe` on Windows.
+By default, commands are executed using a predefined shell (`/bin/sh` on Unix, `cmd.exe` on Windows).
 If you want to use a different shell, you can use the `-S, --shell <SHELL>` option:
 ```sh
 hyperfine --shell zsh 'for i in {1..10000}; do echo test; done'