lttng-ust(3): reorganize the "Context information" section

lttng-ust(3): reorganize the "Context information" section

This patch reorganizes the "Context information" of the
lttng-ust(3) manual page so as to categorize the context fields
into:

* General context fields
* Process context fields
* perf context fields
* Namespace context fields
* Credential context fields

This patch also changes parts of said section to replace some external
manual page references with English nouns for consistency, moving the
references to parentheses.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: Ib93111ab9d252f891fab0a21c106dcf7d33975a3