Fixed typos, highlighted important parts, moved important information to the top (iOS/Android only, Deprecated etc)

[Nikita0x]

Thank you for contributing to RevenueCat/purchases-capacitor. Before pressing the "Create Pull Request" button, please provide the following:

• A description about what and why you are contributing, even if it's trivial.
• Why?
  Because I like RevenueCat and how easy it makes everything to work with subs and all that jazz... Really a lot of work off our shoulders.
• What are you contributing?
  I am contributing some improvements (in my opinion) and some typos to documentation.

Before

After

On the first line I wanted to have important information - if it is Deprecated, iOS or Android only, so that people (and me) sometimes read through the docs of what a method does, when implementing for a specific platform to only discover at the very end, that that platform is not supported. I think it should be on the first line, in the beginning.
Also, I tried to separate it from the main wall of text to visually distinct it from what a method does.
So like you hover over a method - it says "iOS only" and I understand - "Oh, okay. I don't need to read the rest and waste my time since I am working on Android".

Secondly, I added highlight for variables or objects whatever the method returns, what is important. Again to visually distinct it from the docs.

Before

After

Please, point out anything that needs correction, or decline it altogether if you think it is not worth to be added.

• The issue number(s) or PR number(s) in the description if you are contributing in response to those.
  None, idk.

• If applicable, unit tests.
  There is no need for unit tests, I guess? For documentation.