Postdoctoral grants for writing review papers
In Canadian grant parlance, I believe the kind of grant you are talking about is called "knowledge synthesis". The Canadian Institutes of Health Research (CIHR) has some information that describes this kind of grant. However, although such a thing exists, it might not be so readily available:


The page I linked is rather old (probably at least 5 years old), so I don't know if CIHR still funds this kind of research.
I'm not sure how CIHR operates, but I know that SSHRC (for social sciences and humanities) sends out very specific calls from time to time for knowledge synthesis grants specifically for literature reviews (here is a recent example). So, in that case, it is the funding agency that picks the topic, not the researcher.
In general, I don't think the major Canadian funding agencies offer this kind of grant directly to post-docs. Normally, it is for faculty members. Post-doc grants are separate, and I'm not sure if they would be fund literature reviews.


Ultimately, as far as CIHR is concerned, it is best to directly ask the scientific director of the specific institute that covers your area of research. They might be able to answer your questions