Improve description of ShellCommandRequirement.

Author: Peter Amstutz

draft-3/cwl-avro.yml

@@ -2054,6 +2054,10 @@
   name: ShellCommandRequirement
   extends: "#ProcessRequirement"
   doc: |
-    Modify the behavior of CommandLineTool to generate a shell command line
-    instead of a directly executable argument list.  Enables use of shell
-    commands like pipes.
+    Modify the behavior of CommandLineTool to generate a single string
+    containing a shell command line.  Each item in the argument list must be
+    joined into a string separated by single spaces and quoted to prevent
+    intepretation by the shell, unless `CommandLineBinding` for that argument
+    contains `shellQuote: false`.  If `shellQuote: false` is specified, the
+    argument is joined into the command string without quoting, which allows
+    the use of shell metacharacters such as `|` for pipes.