Document the PAGER environment variable (#711)

* Document the PAGER environment variable
Wasn't sure where to put it, I don't think it deserves its own section
This is probably enough, right? Searching for "pager" in the searchbox will find it
Closes #633

* Full default value shown

docs/docs/env-vars.md

@@ -30,3 +30,8 @@ menu: "References"
 
 **Description**: Path to the `kustomize` executable  
 **Default**: `$PATH/kustomize`
+
+### PAGER
+
+**Description**: Pager to use when displaying output. Set to an empty string to disable paging.
+**Default**: `less --RAW-CONTROL-CHARS --quit-if-one-screen --no-init`