Recheck, cover tests, actualize and document disposable's related logic

[AlexInLog]

Maybe revert https://github.com/victimsnino/ReactivePlusPlus/pull/640/files
#662 this one is incorrect partially
Rewrite this one https://discord.com/channels/1014882683895156757/1014882910802804766/1295830271500750898

Currently disposables could be a bit non-intuitive and can be interpreted as subscription but actually they are not. It should be documented properly

[AlexInLog]

#622
#592
#515