Will the 2020r1 be based on 2020r2.5 or will the 2020r2.6 be 2021r1?
In general, most changes that go in to the regular FlexNet Manager Suite Cloud (Flexera One ITAM) updates will also be included the next FlexNet Manager Suite On-premises release. So the upcoming 2021 R1 On-premises release is expected to contain similar changes to what has been included in all of the 2020 R2.x Cloud updates since the previous On-premises (2020 R2) release.
NB. I'm intentionally using somewhat vague language to allow for the possibility that there may always be exceptions. For example, some capabilities may be delivered in an On-premises release but not Cloud updates, or vice versa. But any such exceptions are likely to be relatively rare.
is there any documentation which updates will be made in 2021 R1?
Documentation and information about the FlexNet Manager Suite On-premises 2021 R1 release will be published with the release in a few weeks.
In the meantime, you may like to check out information about capabilities that have been already made available in recent FlexNet Manager Suite Cloud updates as detailed in the FlexNet Manager Release Blog. This will give you a sense of changes that are likely to come in the 2021 R1 On-premises release.