Docs: ensure docs for Dict use full variable names

The following variable switches have been made:

 * `d` has been changed to `dict`
 * `a` has been changed to `dict1`
 * `b` has been changed to `dict2`
 * `d1` has been changed to `dict1`
 * `d2` has been changed to `dict2`

 This fixes #2574
Merge pull request #2575 from Gazler/docs/dict

Docs: ensure docs for Dict use full variable names
Docs: change Dict.merge description to read more naturally

Previously there was an additional "dict" - this is as a result of it
previously saying "dict `a`"
Merge pull request #2576 from Gazler/docs/dict

Docs: change Dict.merge description to read more naturally