HADOOP-13940. Document the missing envvars commands (Contributed by Yiqun Lin via Daniel Templeton)

hadoop-common-project/hadoop-common/src/site/markdown/CommandsManual.md

@@ -223,6 +223,12 @@ Usage: `hadoop CLASSNAME`
 
 Runs the class named `CLASSNAME`. The class must be part of a package.
 
+### `envvars`
+
+Usage: `hadoop envvars`
+
+Display computed Hadoop environment variables.
+
 Administration Commands
 -----------------------
 

hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/site/markdown/MapredCommands.md

@@ -140,6 +140,12 @@ Prints the version.
 
 Usage: `mapred version`
 
+### `envvars`
+
+Usage: `mapred envvars`
+
+Display computed Hadoop environment variables.
+
 Administration Commands
 -----------------------
 

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-site/src/site/markdown/YarnCommands.md

@@ -166,6 +166,12 @@ Usage: `yarn version`
 
 Prints the Hadoop version.
 
+### `envvars`
+
+Usage: `yarn envvars`
+
+Display computed Hadoop environment variables.
+
 Administration Commands
 -----------------------