Improve validation messages for SEO & metadata fields

[SvSven]

The validation labels used for the SEO & metadata fields can be made more clear and consistent.

For example the use of The document title is very important for SEO alongside Meta description is important for SEO is a bit strange.

Similarly Please pay attention to SEO could be written better, for example something along the lines of It is recommended to fill out the SEO fields whenever possible.

Needs input from @BirteThornquist

Easy to fix if we can agree on better texts

[meols]

I see the "yellow triangle" with explanatory text on news and magazine templates, and the texts are OK. But we don't have the "yellow triangles on topic pages. Topic pages are where focus the most on SEO.

[millianapia]

as text is ok, i will move this forward but have created a bug for the warning not being in the top for topic pages #2442