Document the ambiguity of 1-base long seq with QUAL *

This is an extreme edge case likely to never occur, but nevertheless
tool implementors still need to know how to handle it.  Given it *may*
be QUAL 9 or it *may* be QUAL "unknown", we treat it as always unknown.

Fixes samtools#715

SAMv1.tex

@@ -636,9 +636,13 @@ \subsection{The alignment section: mandatory fields}\label{sec:alnrecord}
 \item {\sf QUAL}: ASCII of base QUALity plus 33 (same as the quality
   string in the Sanger FASTQ format). A base quality is the phred-scaled
   base error probability which equals $-10\log_{10}\Pr\{\mbox{base is
-    wrong}\}$. This field can be a `*' when quality is not stored. If
-  not a `*', {\sf SEQ} must not be a `*' and the length of the quality string
-  ought to equal the length of {\sf SEQ}.
+    wrong}\}$.
+  This field can be a `*' when quality is not stored.\footnote{Note an
+  ambiguity exists for the unlikely case of a non-`*' sequence of length 1
+  with base quality 9 (ASCII `*').  Given the ambiguity, the quality
+  should always deemed to be unavailable in this scenario.}
+  If not a `*', {\sf SEQ} must not be a `*' and the length of the quality
+  string ought to equal the length of {\sf SEQ}.
 \end{enumerate}
 
 \subsection{The alignment section: optional fields}\label{sec:alnaux}