Review "Legacy DRL conventions" section wordings

[tkobayas]

in KIE 10.0.0 drools docs, https://kie.apache.org/drools-docs/drools/language-reference/index.html#con-drl-legacy_drl-rules

Legacy DRL conventions

The following Drools Rule Language (DRL) conventions are no longer applicable or optimal in Drools but might be available for backward compatibility.
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gives the impression that function, global and other syntax are deprecated. However, the sections was originally written for RuleUnit. In reality, those syntax are still fully supported with traditional DRL in KIE 10. Review and consider the wordings.