Clarifiy documentation for LZ4F_HEAPMODE

Now it's only about LZ4F_compressFrame(),
and it's only about the Compression Context
(not the hash tables, which used to be integrated).

GitOrigin-RevId: 72b9348f023479b7d9afb4b7c2c8a666e82ff7a8
Change-Id: I24e145d8e66df61ec9e0981b9a71d7ebe3e6dcc2