[TASK] Add unit tests for TYPO3SEARCH markers

Add two unit tests for Indexer, covering content extraction
from between TYPO3SEARCH_begin and TYPO3SEARCH_end markers.

Add note to documentation that it's possible to have multiple
TYPO3SEARCH marker pairs.

Resolves: #74815
Releases: master, 7.6, 6.2
Change-Id: I37c67dfc055dc30698831eef6d0231d929fef957
Reviewed-on: https://review.typo3.org/47175


Reviewed-by: Daniel Goerz <ervaude@gmail.com>
Tested-by: Daniel Goerz <ervaude@gmail.com>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>