Improve description of feature toggles in section describing $GLOBALS['TYPO3_CONF_VARS']['SYS']['feature']

[sypets]

Expected behavior/output

• use the term "Feature toggle(s)" because that is the correct term (relevant for SEO).
• Add a link to description of feature toggles.
• Add explanation where they can be set in the BE

https://docs.typo3.org/m/typo3/reference-coreapi/main/en-us/Configuration/Typo3ConfVars/SYS.html#typo3ConfVars_sys_features

TYPO3 versions

• all versions (which contain this part)