DOC: config: uniformize the naming and description of custom log format args

DOC: config: uniformize the naming and description of custom log format args

A significant number of actions now take arguments that are evaluated as
log-format expressions. Some of them are called "fmt", others "string".
The description of the argument sometimes just says "the log-format
string" or "log format" or "custom log format" etc. Most of them do not
mention the section to visit, and section 8.2 speaking about log-format
is very centric on logs usage (the primary use case), making all of this
very confusing for newcomers.

Since section 8.2.6 is titled "Custom log format" and describes the syntax
to be used with the "log-format" (and other) directives, let's call this
"Custom log format" everywhere and mention section 8.2.6. When the field
was called "string", it was also renamed to "fmt".

It doesn't seem worth backporting this, unless it applies fine.