buildFHSUserEnv: document `profile` parameter

doc/builders/special/fhs-environments.section.md

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         Additional commands to be executed for finalizing the derivation with runner script.
 - `runScript`
         A command that would be executed inside the sandbox and passed all the command line arguments. It defaults to `bash`.
+- `profile`
+        Optional script for `/etc/profile` within the sandbox.
 
 One can create a simple environment using a `shell.nix` like that: