LU-12837 doc: add llapi_changelog* manpages

LU-12837 doc: add llapi_changelog* manpages

This patch adds the following manpages for API changelog interface:
- llapi_changelog_clear.3
- llapi_changelog_fini.3
- llapi_changelog_free.3
- llapi_changelog_get_fd.3
- llapi_changelog_in_buf.3
- llapi_changelog_recv.3
- llapi_changelog_set_xflags.3
- llapi_changelog_start.3

This patch also cleans outdated comments about changelogs in Lustre
API files.

Signed-off-by: Etienne AUJAMES <eaujames@ddn.com>
Test-Parameters: trivial
Change-Id: Ie1481b820661e9e0ce9277ba4c65f51774f1e6bb
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/49084
Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>