DOCS-91: corrections to limits

Author: Sam Kleinman

source/reference/limits.txt

@@ -36,25 +36,17 @@ Namespaces
 .. limit:: Namespace Length
 
    Each namespace, including database and collection name, must be
-   shorter than 628 bytes.
+   shorter than 123 bytes.
 
    .. fix when we know what the actual limit is.
 
 .. _limit-number-of-namespaces:
 .. limit:: Number of Namespaces
 
-   The limitation on the number of namespaces is a function of the
-   size of the namespace file.
+   The limitation on the number of namespaces is the size of the
+   namespace file divided by 628.
 
-   .. ( size of namespace file / 628 ) rounded down.
-
-   By default namespace files are 16 megabytes; however, with the
-   :setting:`nssize` setting, ns files can be no larger than 2
-   gigabytes.
-
-   .. todo:: rephrase above
-
-   A 16 megabyte namespace file can support 24,000 namespaces.
+   A 16 megabyte namespace file can support approximately 24,000 namespaces.
 
 .. _limit-size-of-namespace-file:
 .. limit:: Size of Namespace File
@@ -70,9 +62,8 @@ Indexes
 .. _limit-index-size:
 .. limit:: Index Size
 
-   Indexed items, including their namespace/database, can be *no
-   larger* than 1024 bytes. This value is the indexed content
-   (i.e. the field value.)
+   Indexed items can be *no larger* than 1024 bytes. This value is the
+   indexed content (i.e. the field value, or compound field value.)
 
 .. _limit-number-of-indexes-per-collection:
 .. limit:: Number of Indexes per Collection
@@ -87,7 +78,7 @@ Indexes
    index name is the concatenation of the field names and index
    directions.
 
-   You can explicitly specify an name to :dbcommand:`createIndex` or
+   You can explicitly specify a name to :dbcommand:`createIndex` or
    the :method:`db.collection.ensureIndex()` helper if the default
    index name is too long.
 
@@ -102,6 +93,10 @@ Indexes
    .. see:: :doc:`/tutorial/enforce-unique-keys-for-sharded-collections`
       for an alternate approach.
 
+.. limit:: Number of Indexed Fields in a Compound Index: 
+
+   There can be no more than 31 fields in a compound index.
+
 Replica Sets
 ~~~~~~~~~~~~
 
@@ -137,6 +132,11 @@ Operations
    from the :operator:`$where` function. This is uncommon in
    un-sharded collections.
 
+   The :operator:`$atomic` update modifier does not work in sharded
+   environments.
+
+   :operator:`$snapshot` queries do not work in sharded environments. 
+
 Naming Restrictions
 ~~~~~~~~~~~~~~~~~~~
 
@@ -145,13 +145,13 @@ Naming Restrictions
    The dot (i.e. ``.``) character is not permissible in database
    names.
 
-   Database names are only case sensitive if the underlying file
-   system has case sensitive file names.
+   Database names are case sensitive even if the underlying file
+   system is case insensitive.
 
    .. versionchanged:: 2.2
       For MongoDB instances running on Windows.
 
-      In 2.2 the following characters permissible in database names:
+      In 2.2 the following characters are not permissible in database names:
 
       .. code-block:: none