It seems to me docs/install may not have been updated.
The general rule is:
A version of Lazarus should be compatible with the two most recent FPC releases, at the time of the release of that Lazarus version.
I.e. if at the time Lazarus 2.2.0 was released, the current FPC was 3.2.2, then it should be compatible with 3.2.2 and 3.2.0.
I am not sure, if updates to fixes would break the 3.2.0 compatibility, if in the meantime FPC 3.2.4 was out (and therefore the 2 most recent FPC would be 3.2.4 and 3.2.2).
I don't know what the last Lazarus was that supported 2.6.4. That would have to be from the time that 3.0.0 was the current fpc.