Change method name for doc header button registration to
`registerDocHeaderButtons`, since it strictly follows the PSR-1
camelCase naming.

Resolves: #94235
Releases: master
Change-Id: Ib6bc5af33c11ec96db4aeb77364a2683f52bccf8
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/69325


Tested-by: core-ci <typo3@b13.com>
Tested-by: Simon Gilli <typo3@gilbertsoft.org>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Simon Gilli <typo3@gilbertsoft.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>