Clarify PileupColumn::add_indels documentation
In particular, explain how to provide a reference sequence file;
`AlignmentFile(..., reference_filename = ...)` is not connected to
this machinery. Addresses
@@ -2952,9 +2952,10 @@ cdef class PileupColumn:
add_indels : bool
- If True, add bases for bases inserted into the reference and
- 'N's for base skipped from the reference. If a reference sequence
- is given, add the actual bases.
+ If True, add bases for bases inserted into or skipped from the
+ reference. The latter requires a reference sequence file to have
+ been given, e.g. via `pileup(fastafile = ...)`. If no reference
+ sequence is available, skipped bases are represented as 'N's.
Returns
-------
