Fix ambiguity about whether final bit padding must be zero

* Currently, the wording makes it clear that fill bits must be zero if
ISLASTEMPTY is true. However, it says nothing about whether the fill bits must
be zero if the meta-block is the last, but is also non-empty. The behavior of
the reference implementation is that it must be zero. Thus, move the requirement
of fill bits being zero to the ISLAST description since that covers ISLASTEMPTY.

* Also, make the description of ISUNCOMPRESSED consistent with the way meta
data are described.

docs/draft-alakuijala-brotli-07.nroff

@@ -1441,14 +1441,14 @@ meta-block is the last one. The format of the meta-block header is
 the following:
 
 .nf
-   1 bit:  ISLAST, set to 1 if this is the last meta-block
+   1 bit:  ISLAST, set to 1 if this is the last meta-block;
+           at the end of this meta-block, any remaining bits in
+           the last byte of the compressed stream are filled with
+           zeros (if the fill bits are not zero, then the stream
+           should be rejected as invalid)
    1 bit:  ISLASTEMPTY, if set to 1, the meta-block is empty;
            this field is only present if ISLAST bit is set -- if
-           it is 1, then the meta-block and the brotli stream ends
-           at that bit, with any remaining bits in the last byte
-           of the compressed stream filled with zeros (if the
-           fill bits are not zero, then the stream should be
-           rejected as invalid)
+           it is 1, then the meta-block ends at that bit
    2 bits: MNIBBLES, # of nibbles to represent the uncompressed
            length, encoded with the following fixed-length code:
 
@@ -1488,12 +1488,15 @@ the following:
            nibble is all zeros, then the stream should be
            rejected as invalid)
 
-   1 bit:  ISUNCOMPRESSED, if set to 1, any bits of compressed
-           data up to the next byte boundary are ignored, and
-           the rest of the meta-block contains MLEN bytes of
-           literal data; this field is only present if the
-           ISLAST bit is not set (if the ignored bits are not
-           all zeros, the stream should be rejected as invalid)
+   1 bit:  ISUNCOMPRESSED, if set to 1, the meta-block contains
+           uncompressed data; this field is only present if ISLAST
+           bit is set -- if it is 1, then the rest of the
+           meta-block has the following format:
+
+              0 - 7 bits: fill bits until the next byte boundary,
+                      must be all zeros
+
+              MLEN bytes of uncompressed data
 
    1-11 bits: NBLTYPESL, # of literal block types, encoded with
            the following variable length code (as it appears in