DOCS-15103: type 7 (compressed column) (#84)

* fix formatting in spec file

* DOCS-15103: add details about column type 7

* HE fix

(cherry picked from commit 38dfc47)

spec.html

@@ -343,6 +343,10 @@ <h3>Notes</h3>
     <li>Max key - Special type which compares higher than all other possible BSON element values.</li>
     <li>Generic binary subtype - This is the most commonly used binary subtype and should be the 'default' for drivers
       and tools.</li>
+    <li>Compressed BSON Column - Compact storage of BSON data. This data
+      type uses delta and delta-of-delta compression and
+      run-length-encoding for efficient element storage. Also has an
+      encoding for sparse arrays containing missing values.</li>
     <li>The BSON "binary" or "BinData" datatype is used to represent
       arrays of bytes. It is somewhat analogous to the Java notion of a
       ByteArray. BSON binary values have a <em>subtype</em>. This is used to