DOCS-1708: Documentation isn't clear on what a 'blocking index build' is

source/core/indexes.txt

@@ -633,20 +633,25 @@ affect how MongoDB builds the indexes.
 Background Construction
 ~~~~~~~~~~~~~~~~~~~~~~~
 
-By default, creating an index is a blocking operation. Building an
-index on a large collection of data can take a long
-time to complete. To resolve this issue, the background option can
-allow you to continue to use your :program:`mongod` instance during
-the index build.
-
-For example, to create an index in the background of the ``zipcode``
-field of the ``people`` collection you would issue the following:
+By default, creating an index is a blocking operation. When building
+an index, the databases that hold those collections are not available
+for any read or write operations until the index is totally built.
+Any operation requiring a read or write lock on all databases (e.g.
+:command:`listDatabases`) will wait for the index to complete building.
+
+For potentially long running index building operations you can use the
+``background`` operation so that your MongoDB instance remains available
+during the index building operation. For example, to create an index in
+the background of the ``zipcode`` field of the ``people`` collection you
+would issue the following:
 
 .. code-block:: javascript
 
    db.people.ensureIndex( { zipcode: 1}, {background: true} )
 
 By default, ``background`` is ``false`` for building MongoDB indexes.
+Setting ``background`` to ``true`` makes the index creation a
+non-blocking operation.
 
 You can combine the background option with other options, as in the
 following:

source/tutorial/build-indexes-in-the-background.txt

@@ -8,11 +8,18 @@ Build Indexes in the Background
 .. default-domain:: mongodb
 
 By default, MongoDB builds indexes in the foreground, which means that
-these indexes block all other read and write operations to the
-database while the index builds. :ref:`Background index construction
-<index-creation-background>` allows read and write operations to
-continue while building the index; however, these index builds take
-longer to complete and result in a larger index.
+these no read and write operations to the database can occur while the
+index builds. Also, no operations that need a read or write lock on all
+databases (e.g. :command:`listDatabases`) can occur during this time as
+well.
+
+:ref:`Background index construction <index-creation-background>` allows
+read and write operations to continue while building the index.
+
+.. note::
+
+   These index builds take longer to complete and result in a larger
+   index.
 
 After the index finishes building, MongoDB treats indexes built in the
 background the same as any other index.