Fix document : drl functions in a different package cannot be imported

Issue Description:

Drl functions in a different package cannot be imported.

package com.sample

import function com.func.isOdd;

rule Hello1
...

results in

Error importing : 'com.func.isOdd'
Rule Compilation error : [Rule name='Hello1']
	Only a type can be imported. com.func.isOdd resolves to a package

The document is misleading and should be fixed.

https://docs.drools.org/7.73.0.Final/drools-docs/html_single/#drl-functions-con_drl-rules
https://docs.drools.org/latest/drools-docs/docs-website/drools/language-reference/index.html#_legacy_functions_in_drl

On the other hand, Java static methods in a different package can be imported.

Acceptance Criteria:

• Fix description about "import function" in docs 7.x and 8. Write that drl functions in a different package cannot be imported.