Fixup IUPAC alleles reference

Author: jeromekelleher

docs/api.md

@@ -60,6 +60,8 @@ that have already been generated.
 ```
 
 ```{eval-rst}
+.. data:: IUPAC_ALLELES
+
 .. autofunction:: decode_flags
 
 .. autofunction:: flags_summary

sc2ts/core.py

@@ -22,7 +22,11 @@
 # NOTE!! This string is also used in the jit module where it's
 # hard-coded into a numba function, so if this ever changes
 # it needs to be updated there also!
+
 IUPAC_ALLELES = "ACGT-RYSWKMBDHV."
+"""
+The allele-integer encoding used by sc2ts.
+"""
 
 NODE_IS_MUTATION_OVERLAP = 1 << 21
 NODE_IS_REVERSION_PUSH = 1 << 22