Update documentation for overloading methods

[Adrian-Hirt]

From ruby/gem_rbs_collection#16:

I did not find the ... triple dot syntax for overloading methods, e.g. when defining a method multiple times.

I added the DuplicatedMethodDefinitionError to the docs/sigs.md, such that users that encounter that error have some pointers on what to do.

I also added the ... syntax to the _method-types_ in docs/syntax.md, but I'm not sure if that is 100% the correct place.
In some places, the triple-dot (ellipsis) is used as a placeholder for "other may follow", which can be a bit confusing, and as such, I replaced it by etc..
However, at two places, I was not entirely sure if the ... is a mere placeholder, or functions as the overloading operator:

1. In the Members section, for the _method-name_:

_method-name_ ::= ...
                | /`[^`]+`/

2. In the Declarations section, for the _global-name_:

_global-name_ ::= /$[a-zA-Z]\w+/ | ...

I'd appreciate it if someone could give some feedback, what function the ... has at that place, I can then gladly add another commit to make it more clear.

Regards, and thank you all for your work

Adrian

[soutaro]

Thanks!