Added append equals, merge append. Fixed creating numeric arrays

pkg/yqlib/doc/headers/Add.md

@@ -2,3 +2,5 @@ Add behaves differently according to the type of the LHS:
 - arrays: concatenate
 - number scalars: arithmetic addition (soon)
 - string scalars: concatenate (soon)
+
+Use `+=` as append assign for things like increment. `.a += .x` is equivalent to running `.a |= . + .x`.

pkg/yqlib/doc/headers/Multiply.md

@@ -2,6 +2,8 @@ Like the multiple operator in `jq`, depending on the operands, this multiply ope
 
 Upcoming versions of `yq` will add support for other types of multiplication (numbers, strings).
 
+To concatenate when merging objects, use the `*+` form (see examples below). This will recursively merge objects, appending arrays when it encounters them.
+
 Note that when merging objects, this operator returns the merged object (not the parent). This will be clearer in the examples below.
 
 ## Merging files