Document different increase pattern for -P

Document values of `-P` increasing as powers of 2 and following other
patterns.

Fixes #403

src/app.rs

@@ -113,7 +113,11 @@ fn build_app() -> App<'static, 'static> {
                     "Perform benchmark runs for each value in the range MIN..MAX. Replaces the \
                      string '{VAR}' in each command by the current parameter value.\n\n  \
                      Example:  hyperfine -P threads 1 8 'make -j {threads}'\n\n\
-                     This performs benchmarks for 'make -j 1', 'make -j 2', …, 'make -j 8'.",
+                     This performs benchmarks for 'make -j 1', 'make -j 2', …, 'make -j 8'.\n\n\
+                     To have the value increase following different patterns, use shell arithmetics.\n\n  \
+                     Example: hyperfine -P size 0 3 'sleep $((2**{size}))'\n\n\
+                     This performs benchmarks with power of 2 increases: 'sleep 1', 'sleep 2', 'sleep 4', …\n\
+                     The exact syntax may vary depending on your shell and OS."
                 ),
         )
         .arg(