Thanks for your posting and for recommending TestRail, that’s really appreciated!
TestRail doesn’t have a built-in requirement management module and we recommend using the References field for this to integrate with external tools (such as a Wiki, issue tracker or dedicated requirement management tool). Most teams that manage requirements already have a tool in place for this and we believe that having good integration support with these tools through the References field is a better approach than adding requirements directly to TestRail itself.
As an aside, the next TestRail version will introduce several Reference/Requirement related reports that drastically improve the usefulness and visibility of linked references (e.g. showing you their test case coverage, test results or associated defects, e.g.).
I hope this helps and please let me know in case you have any further questions.