We really need to break the monolithic samtools manpage into sub-pages, but the samtools manpage does describe the pileup format and is likely to be more accurate than third-party rewrites. From http://www.htslib.org/doc/samtools.html
In the pileup format (without -u or -g), each line represents a
genomic position, consisting of chromosome name, 1-based coordinate,
reference base, the number of reads covering the site, read bases,
base qualities and alignment mapping qualities. Information on match,
mismatch, indel, strand, mapping quality and start and end of a read
are all encoded at the read base column. At this column, a dot stands
for a match to the reference base on the forward strand, a comma for a
match on the reverse strand, a '>' or '<' for a reference skip,
ACGTN for a mismatch on the forward strand and acgtn for a
mismatch on the reverse strand. A pattern \+[0-9]+[ACGTNacgtn]+
indicates there is an insertion between this reference position and
the next reference position. The length of the insertion is given by
the integer in the pattern, followed by the inserted sequence.
Similarly, a pattern -[0-9]+[ACGTNacgtn]+ represents a deletion from
the reference. The deleted bases will be presented as * in the
following lines. Also at the read base column, a symbol ^ marks the
start of a read. The ASCII of the character following ^ minus 33
gives the mapping quality. A symbol $ marks the end of a read
segment
A sequence matching the regular expression \+[0-9]+[ACGTNacgtn]+ denotes an insertion of one or more bases starting from the next position
A sequence matching the regular expression -[0-9]+[ACGTNacgtn]+ denotes a deletion of one or more bases starting from the next position
(asterisk) is a placeholder for a deleted base in a multiple basepair deletion that was mentioned in a previous line by the -[0-9]+[ACGTNacgtn]+ notation
