Write documentation for classes and methods

[ivanhercaz]

Hi!
This package is awesome and really great, I need it and I enjoy a lot to use it, so I want to do my bit! I started to write documentation (dartdoc comments) for classes and methods of the package.

I leave this PR as a draft until I think it is enough or maybe everything it's documented! If I see that it's going to take a long time, I will remove the draft status of this PR and in the future I will work in another one.

Please, review the documentation and leave comments if it is necessary, because the best documentation is the one that it is reviewed for more than one! ❤️

I will always try to write the docs based on the recommendation of Effective Dart: Documentation, to be clear but also concise about what it is necessary.

[ogabrielinacio]

this is good enough for me, thank you very much, it will help us increase pub points on pub.dev :)

About the first line

How about something like this?

An abstract class that must be implemented in a higher layer of software, which must have at least the following methods

[ivanhercaz]

About the first line

How about something like this?

An abstract class that must be implemented in a higher layer of software, which must have at least the following methods

I thought about using "abstract class" but I feel a bit redundant because in the generated documentation the signature already indicates it is an abstract class.

Now it is really late but soon I will review your comment and rethinking the abstract class documentation.

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As a note, as this PR is going to be a bit huge, it could be really useful if we make the reviews per line using the review system and let the comments for general reviews, in that way we could have a more organized environment with threads per issues/doubts/improvements in the contributions.

What do you think?