API.md : clarify max buffer length configuration

related to https://github.com/dailymotion/hls.js/issues/158
relatred to https://github.com/dailymotion/hls.js/issues/58

API.md

@@ -184,6 +184,7 @@ configuration parameters could be provided to hls.js upon instantiation of Hls O
       debug : false,
       autoStartLoad : true,
       maxBufferLength : 30,
+      maxMaxBufferLength : 600,
       maxBufferSize : 60*1000*1000,
       liveSyncDurationCount : 3,
       liveMaxLatencyDurationCount: 10,
@@ -232,10 +233,24 @@ a logger object could also be provided for custom logging : ```config.debug=cust
 (default 30s)
 
 maximum buffer Length in seconds. if buffer length is/become less than this value, a new fragment will be loaded.
+this is the guaranteed buffer length hls.js will try to reach, regardless of maxBufferSize.
+
 #### ```maxBufferSize```
 (default 60 MB)
 
-maximum buffer size in bytes. if buffer size upfront is bigger than this value, no fragment will be loaded.
+'minimum' maximum buffer size in bytes. if buffer size upfront is bigger than this value, no fragment will be loaded.
+
+#### ```maxMaxBufferLength```
+(default 600s)
+
+maximum buffer Length in seconds. hls.js will never exceed this value. even if maxBufferSize is not reached yet.
+
+hls.js tries to buffer up to a maximum number of bytes (60 MB by default) rather than to buffer up to a maximum nb of seconds.
+this is to mimic the browser behaviour (the buffer eviction algorithm is starting after the browser detects that video buffer size reaches a limit in bytes)
+
+config.maxBufferLength is the minimum guaranteed buffer length that hls.js will try to achieve, even if that value exceeds the amount of bytes 60 MB of memory.
+maxMaxBufferLength acts as a capping value, as if bitrate is really low, you could need more than one hour of buffer to fill 60 MB....
+
 
 #### ```liveSyncDurationCount```
 (default 3)