tracing: Document tracefs gid mount option

Commit ee7f3666995d ("tracefs: Have new files inherit the ownership of
their parent") and commit 48b27b6b5191 ("tracefs: Set all files to the
same group ownership as the mount option") introduced a new gid mount
option that allows specifying a group to apply to all entries in tracefs.

Document this in the tracing readme.

Cc: Eric Sandeen <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Ali Zahraee <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: David Howells <[email protected]>
Cc: Masami Hiramatsu <[email protected]>
Link: https://lore.kernel.org/[email protected]
Signed-off-by: Kalesh Singh <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

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