meson: rename conflicting target names

The update-man-rules and update-dbus-docs targets are both declared in the
main meson.build and man/meson.build, so we cannot build the target with
'meson compile' command:
====
$ meson compile update-man-rules
INFO: autodetecting backend as ninja

ERROR: Can't invoke target `update-man-rules`: ambiguous name.Add target type and/or path:
- ./man/update-man-rules:custom
- ./update-man-rules:run
====
Let's rename the targets declared in man/meson.build.

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