DOCS-12251: isMaster - fix compression field description

Author: Kay Kim

source/reference/command/isMaster.txt

@@ -150,23 +150,47 @@ roles:
 .. data:: isMaster.compression
 
    .. versionadded:: 3.4
+
+   An array listing the compression algorithms used or available for
+   use (i.e. common to both the client and the :binary:`~bin.mongod` or
+   :binary:`~bin.mongos` instance) to compress the communication
+   between the client and the :binary:`~bin.mongod` or
+   :binary:`~bin.mongos` instance.
+
+   The field is only available if compression is used. For example:
+
+   - If the :binary:`~bin.mongod` is enabled to use both the
+     ``snappy,zlib`` compressors and a client has specified ``zlib``,
+     the :data:`~isMaster.compression` field would contain:
+
+     .. code-block:: javascript
 
-   .. versionchanged:: 3.6 In MongoDB 3.6, :data:`~isMaster.compression`
-      appears by default. Previously, :data:`~isMaster.compression` only
-      appeared if the network connection was compressed.
+        "compression": [ "zlib" ]
+
+   - If the :binary:`~bin.mongod` is enabled to use both the
+     ``snappy,zlib`` compressors and a client has specified ``zlib,snappy``,
+     the :data:`~isMaster.compression` field would contain:
+
+     .. code-block:: javascript
 
-   An array listing the compression algorithms used to compress the
-   current connection's network communication between the
-   :binary:`~bin.mongod` or :binary:`~bin.mongos` and other members of the
-   deployment or the :binary:`~bin.mongo` shell.
+        "compression": [ "zlib", "snappy" ]
 
-   For a :binary:`~bin.mongod` or :binary:`~bin.mongos` using the ``snappy``
-   compressor , the :data:`~isMaster.compression` field would resemble
-   the following:
+   - If the :binary:`~bin.mongod` is enabled to use  the
+     ``snappy`` compressor and a client has specified ``zlib,snappy``,
+     the :data:`~isMaster.compression` field would contain :
 
-   .. code-block:: sh
+     .. code-block:: javascript
 
-      "compression": [ "snappy" ]
+        "compression": [ "snappy" ]
+
+   - If the :binary:`~bin.mongod` is enabled to use  the ``snappy``
+     compressor and a client has specified ``zlib`` or the client has
+     specified no compressor, the field is omitted.
+
+   That is, if the client does not specify compression or if the client
+   specifies a compressor not enabled for the connected
+   :binary:`~bin.mongod` or :binary:`~bin.mongos` instance, the field
+   does not return.
 
 .. data:: isMaster.saslSupportedMechs