docs: ftrace: Clarify the RAM impact of buffer_size_kb

The current text could mislead the user into believing that the number
of pages allocated by each CPU ring buffer is calculated by the round
up of the division: buffer_size_kb / PAGE_SIZE.

Clarifies that a few extra pages may be allocated to accommodate buffer
management meta-data.

Suggested-by: Steven Rostedt (VMware) <[e-mail]>
Suggested-by: Joel Fernandes (Google) <[e-mail]>
Reviewed-by: Steven Rostedt (VMware) <[e-mail]>
Reviewed-by: Joel Fernandes (Google) <[e-mail]>
Signed-off-by: Frank A. Cancio Bello <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/trace/ftrace.rst

···
 	CPU buffer and not total size of all buffers. The
 	trace buffers are allocated in pages (blocks of memory
 	that the kernel uses for allocation, usually 4 KB in size).
-	If the last page allocated has room for more bytes
+	A few extra pages may be allocated to accommodate buffer management
+	meta-data. If the last page allocated has room for more bytes
 	than requested, the rest of the page will be used,
 	making the actual allocation bigger than requested or shown.
 	( Note, the size may not be a multiple of the page size