UPSTREAM: tracing/histogram: Document expression arithmetic and constants

Histogram expressions now support division, and multiplication in
addition to the already supported subtraction and addition operators.

Numeric constants can also be used in a hist trigger expressions
or assigned to a variable and used by refernce in an expression.

Link: https://lkml.kernel.org/r/20211025200852.3002369-9-kaleshsingh@google.com



Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
Reviewed-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
(cherry picked from commit 2d2f6d4b)

Bug: 146055070
Bug: 145972256
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
Change-Id: Iaf9609e3b0373414280f6c8aae638c034797bbf2