Consistent form_class availability in Member Management forms

[jHards]

Is your feature request related to a problem? Please describe.
Under the Member Management docs, the following forms are listed but not all state that they can accommodate the form_class parameter:

• {exp:member:registration_form}
• {exp:member:login_form}
• {exp:member:logout_form}
• {exp:member:forgot_password_form}
• {exp:member:forgot_username_form}
• {exp:member:reset_password_form}
• {form_declaration} used in {exp:member:memberlist} and {exp:member:member_search}
• {exp:member:edit_profile}
• {exp:member:edit_avatar}

Describe the solution you'd like
I'll submit a pull request shortly for the v6 branch, no obvious reason this should be v7 only.

Teachability, Documentation, Adoption, Migration Strategy
Existing docs pages should be updated to include the form_class parameter, wording is already available on the other pages.

• I am capable and would like to work on implementation of this feature if it is considered.

[jHards]

On closer inspection, it seems that many of the tags above already can accommodate the name, id and class parameters but they are not listed as parameters in the docs.

The following tags are still outstanding:

• {form_declaration} used in {exp:member:memberlist}
• {exp:member:edit_profile}
• {exp:member:edit_avatar}