Are there any way to search in fossil repository for
Fossil's documentation claims there is a search command available in UI: /search but I cannot figure out how to use it.
In versions before 1.31, the search command only searched check-in comments.
As of 1.31, it also searches tickets, wiki, and documentation contents. But it (still) does not search source code, unless you specify your source files as searchable documentation.
As Registered User rightly says, searching needs to be enabled explicitly for each repository. See the help for the srchsetup page, and the fts-config command.
(The Fossil website usually runs a recent version from Fossil's own trunk branch. That's why it sometimes shows features that aren't present (yet) in the most recent binaries).