Update a11y note on acc-name rules

[WilcoFiers]

There are a few rules with the following note:

Note: Testing that the accessible name is descriptive is not part of this rule and must be tested separately.

This should be renamed to:

Note: Testing that the accessible name describes the purpose of the element is not part of this rule and must be tested separately.

This note is used in the following rules:

• 97a4e1
• 7d6734
• e086e5
• cae760
• 23a2a8
• 59796f
• c487ae

[Jym77]

Closed by #1216.