Expand on documentation for compatbits (#63)

Make it more clear exactly what this function promises.

src/aminoacid.jl

@@ -198,6 +198,11 @@ gap(::Type{AminoAcid}) = AA_Gap
     compatbits(aa::AminoAcid)
 
 Return the compatibility bits of `aa` as `UInt32`.
+The resulting `UInt32` has one bit set per amino acid
+it is compatible with.
+
+For example, `J` is compatible with `I` (bit 10) and `L` (bit 11),
+and so is `0x00000600`.
 
 Examples
 --------
@@ -206,9 +211,17 @@ Examples
 julia> compatbits(AA_A)
 0x00000001
 
+julia> compatbits(AA_E)
+0x00000040
+
 julia> compatbits(AA_J)
 0x00000600
 
+julia> compatbits(AA_X)
+0x003fffff
+
+julia> compatbits(AA_Gap)
+0x00000000
 ```
 """
 compatbits(aa::AminoAcid) = @inbounds compatbits_aa[encoded_data(aa) + 1]

src/nucleicacid.jl

@@ -460,6 +460,11 @@ end
     compatbits(nt::NucleicAcid)
 
 Return the compatibility bits of `nt` as `UInt8`.
+The resulting `UInt8` has the lower four bits set
+if `nt` is compatible with `A`, `C`, `G` and `T/U`, respectively.
+
+Hence, `RNA_Gap` is `0x00` (not compatible with any nucleotide),
+and `DNA_W` is `0x09` (compatible with `A` and `T`)
 
 Examples
 --------
@@ -474,6 +479,11 @@ julia> compatbits(DNA_C)
 julia> compatbits(DNA_N)
 0x0f
 
+julia> compatbits(DNA_W)
+0x09
+
+julia> compatbits(RNA_Gap)
+0x00
 ```
 """
 @inline function compatbits(nt::NucleicAcid)