added documentation for the seekable format

and notably provide additional context for the
Maximum Frame Size parameter.

requested by @P-E-Meunier
at https://github.com/facebook/zstd/commit/1df9f36c6c6cea08778d45a4adaf60e2433439a3#commitcomment-103856979.

contrib/seekable_format/zstd_seekable.h

@@ -48,10 +48,19 @@ typedef struct ZSTD_seekTable_s ZSTD_seekTable;
 *
 *  Use ZSTD_seekable_initCStream() to initialize a ZSTD_seekable_CStream object
 *  for a new compression operation.
-*  `maxFrameSize` indicates the size at which to automatically start a new
-*  seekable frame.  `maxFrameSize == 0` implies the default maximum size.
-*  `checksumFlag` indicates whether or not the seek table should include frame
-*  checksums on the uncompressed data for verification.
+*  - `maxFrameSize` indicates the size at which to automatically start a new
+*            seekable frame.
+*            `maxFrameSize == 0` implies the default maximum size.
+*            Smaller frame sizes allow faster decompression of small segments,
+*            since retrieving a single byte requires decompression of
+*            the full frame where the byte belongs.
+*            In general, size the frames to roughly correspond to
+*            the access granularity (when it's known).
+*            But small sizes also reduce compression ratio.
+*            Avoid really tiny frame sizes (< 1 KB),
+*            that would hurt compression ratio considerably.
+*  - `checksumFlag` indicates whether or not the seek table should include frame
+*            checksums on the uncompressed data for verification.
 *  @return : a size hint for input to provide for compression, or an error code
 *            checkable with ZSTD_isError()
 *