DOCS-517: increase warning about js and eval

Author: Sam Kleinman

source/applications/server-side-javascript.txt

@@ -7,7 +7,11 @@ Server-side JavaScript
 MongoDB supports server-side execution of JavaScript code using various
 methods.
 
-.. note:: The JavaScript code execution takes a JavaScript lock.
+.. note:: 
+
+   The JavaScript code execution takes a JavaScript lock: each
+   :program:`mongod` can only execute a single JavaScript operation at
+   a time.
 
 .. _server-side-map-reduce:
 
@@ -38,9 +42,13 @@ bottleneck if performing these operations on the client-side.
 
 .. warning::
 
-   By default, :dbcommand:`eval` command requires a write lock. As such
-   :dbcommand:`eval` will block all other read and write operations
-   while it runs.
+   By default, :dbcommand:`eval` command requires a write lock. As
+   such :dbcommand:`eval` will block all other read and write
+   operations while it runs. Because only a single JavaScript process
+   can run at a time, *do not* run :dbcommand:`mapReduce`,
+   :dbcommand:`group`, queries with the :operator:`$where` or any
+   other operation that requires JavaScript execution within
+   :dbcommand:`eval` operations.
 
 See :dbcommand:`eval` command and :method:`db.eval()` documentation for
 more information, including examples.