Update timeout parameter to include suffix descriptions

Author: davidplowman

documentation/asciidoc/computers/camera/rpicam_options_common.adoc

@@ -89,9 +89,19 @@ Alias: `-t`
 
 Default value: 5000 milliseconds (5 seconds)
 
-Specify how long the application runs before closing. This applies to both video recording and preview windows. When capturing a still image, the application shows a preview window for `timeout` milliseconds before capturing the output image.
+Specify how long the application runs before closing. This value is interpreted as a number of milliseconds unless an optional suffix is used to change the unit. The suffix may be one of:
 
-To run the application indefinitely, specify a value of `0`.
+* `min` - minutes
+* `s` or `sec` - seconds
+* `ms` - milliseconds (the default if no suffix used)
+* `us` - microseconds
+* `ns` - nanoseconds.
+
+This time applies to both video recording and preview windows. When capturing a still image, the application shows a preview window for the length of time specified by the `timeout` parameter before capturing the output image.
+
+To run the application indefinitely, specify a value of `0`. Floating point values are also permitted.
+
+Example: `rpicam-hello -t 0.5min` would run for 30 seconds.
 
 ==== `preview`