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) <rostedt@goodmis.org>
Suggested-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Frank A. Cancio Bello <frank@generalsoftwareinc.com>
Link: https://lore.kernel.org/r/6f33be5f3d60e5ffc061d8d2b329d3d3ccf22a8c.1577231751.git.frank@generalsoftwareinc.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/trace/ftrace.rst

@@ -187,7 +187,8 @@ of ftrace. Here is a list of some of the key files:
 	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