Update the remaining manual pages for LTTng-tools 2.13

Update the remaining manual pages for LTTng-tools 2.13

This patch updates the remaining manual pages for LTTng-tools 2.13.

This patch:

* Improves the consistency of some command descriptions in
  `configure.ac`.

* Adds `common-daemon-cfg.txt` which is a section explaining how to
  configure a daemon (session or relay).

  lttng-sessiond(8) and lttng-relayd(8) include this file.

* Adds `lttng-concepts.7.txt` which is an adapted copy of the
  "Core concepts" section of the online LTTng Documentation.

  This centralizes all the LTTng theory into a single manual page
  instead of having this information split into multiple lttng(1)
  command manual pages.

  Many manual pages now refer to lttng-concepts(7), making it possible
  to cut a lot of text in those.

* Updates existing manual pages to:

  * Have a style and voice which is more consistent with the LTTng
    Documentation (website) for 2.13.

  * Fix various terminology ambiguities.

  * Use more textual variables and lists to explain more complex logic
    and processes.

  * Always use the same pattern to specify the behaviour of an lttng(1)
    command depending on the `SESSION` argument or the `--session`
    option.

  * For the commands which can perform more than one task, list their
    available tasks at the beginning of the "DESCRIPTION" section.

  * For some lttng(1) commands which can operate on all tracing sessions
    (for example, lttng-clear(1) and lttng-destroy(1)), always indicate
    that they target all your Unix user's tracing sessions or, if your
    Unix user is `root`, the tracing sessions of all the Unix users
    within the root session daemon.

  * Clean the "SEE ALSO" sections.

  * Always have "LTTng" in the "NAME" section of a manual page.

  More specifically:

  lttng-create(1):
      * Clarify the tracing session modes.

      * Clarify how the command adds (or not) a snapshot output for a
        snapshot mode tracing session.

      * Specify that `--output=DIR` is equivalent to
        `--set-url=file://DIR`.

  lttng-enable-channel(1):
      Include the `--discard`, `--buffers-uid`, and `--buffers-global`
      options in the "SYNOPSIS" section even if they are the current
      defaults.

  lttng-list(1):
      Explain what this command does exactly using a tree of options
      and arguments.

  lttng-load(1):
      Clarify how LTTng finds tracing session configurations.

  lttng-relayd(8):
      * Document the missing `--group` option.

      * Rework the text in general.

      * Add a daemon configuration section with an INI file example.

      * Add more cross-references between options and equivalent
        environment variables.

  lttng-rotate(1):
      Specify that the `rotate-session` trigger action can also rotate
      a tracing session.

  lttng-save(1):
      Clarify the output path.

  lttng-sessiond(8):
      Add more cross-references between options and equivalent
      environment variables.

  lttng-shapshot(1):
      * Clarify everything related to the snapshot output of a tracing
        session, including when and how the lttng-create(1) command adds
        an initial snapshot output.

      * Specify that the `snapshot-session` trigger action can also take
        a snapshot of a tracing session.

  lttng-track(1):
  lttng-untrack(1):
      * Simply refer to allowing processes to record events and to
        process attribute inclusion sets instead of using the vague
        "tracker" terminology.

      * Restate that those commands control an implicit condition of
        a recording event rule, as per lttng-concepts(7).

      * Improve the documentation of each inclusion set selection
        option.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Iac7498ee979fe077f0927a9b8335f6c07f203989