Document that proc named fooTask is created for every foo task [backport] (#14187)

Ref: https://irclogs.nim-lang.org/01-05-2020.html#15:18:03
(cherry picked from commit 0880f118d3)

lib/system/nimscript.nim

@@ -384,11 +384,26 @@ when not defined(nimble):
   template `==?`(a, b: string): bool = cmpIgnoreStyle(a, b) == 0
   template task*(name: untyped; description: string; body: untyped): untyped =
     ## Defines a task. Hidden tasks are supported via an empty description.
+    ##
     ## Example:
     ##
     ## .. code-block:: nim
     ##  task build, "default build is via the C backend":
     ##    setCommand "c"
+    ##
+    ## For a task named ``foo``, this template generates a ``proc`` named
+    ## ``fooTask``.  This is useful if you need to call one task in
+    ## another in your Nimscript.
+    ##
+    ## Example:
+    ##
+    ## .. code-block:: nim
+    ##  task foo, "foo":        # > nim foo
+    ##    echo "Running foo"    # Running foo
+    ##
+    ##  task bar, "bar":        # > nim bar
+    ##    echo "Running bar"    # Running bar
+    ##    fooTask()             # Running foo
     proc `name Task`*() =
       setCommand "nop"
       body