Add defaults convention to example code standards (#1182)

* Add defaults convention to example code standards

Co-authored-by: Dongdong Tian <[email protected]>

Author: Meghan Jones

CONTRIBUTING.md

@@ -187,8 +187,11 @@ When editing documentation, use the following standards to demonstrate the examp
    (two asterisks) on both sides. The argument description should be in *italicized*
    with \* (single asterisk) on both sides.
    Examples: `**+l**\ *label*` results in **+l***label*, `**05m**` results in **05m**.
-3. Optional arguments are placed wrapped with [ ] (square brackets).
+3. Optional arguments are wrapped with [ ] (square brackets).
 4. Arguments that are mutually exclusive are separated with a | (bar) to denote "or".
+5. Default arguments for parameters and configuration settings are wrapped
+   with [ ] (square brackers) with the prefix "Default is". Example: [Default is
+   **p**].
 
 ## Contributing Code