ctree: Add documentation for COMPRESS_*

The defrag_range ioctl can accept a compression type as parameter. Add
some comments to make these values show up in the generated
documentation.

Author: knorrie

btrfs/ctree.py

@@ -283,10 +283,10 @@ def _struct_format(s):
     FT_XATTR: 'XATTR',
 }
 
-COMPRESS_NONE = 0
-COMPRESS_ZLIB = 1
-COMPRESS_LZO = 2
-COMPRESS_ZSTD = 3
+COMPRESS_NONE = 0  #: no compression at all
+COMPRESS_ZLIB = 1  #: zlib compression
+COMPRESS_LZO = 2  #: lzo compression
+COMPRESS_ZSTD = 3  #: zstd compression
 
 _compress_type_str_map = {
     COMPRESS_NONE: 'none',