Add upsert argument to append

[DrNickClarke]

Is your feature request related to a problem? Please describe.
When append is called on a symbol that does not exist, it automatically creates the symbol. This is similar to the upsert features of update. This is not mentioned in the docstring.

Describe the solution you'd like
The append docstring should include a section that mentions the above.

[DrNickClarke]

This is actually the default for append - to create the symbol if it does not already exist. So not needed