RP conventions features update

[guardrex]

mvc/razor-pages/razor-pages-convention-features.md
https://docs.microsoft.com/en-us/aspnet/core/mvc/razor-pages/razor-pages-convention-features

IPageHandlerModelConvention

[guardrex]

Cross-reference:

• Make IParameterModelConvention execute for page handler parameters aspnet/Mvc#6935
• aspnet/Mvc@dbff416#diff-c3c4b436d308486910e2efb7719c2d7a

[guardrex]

@pranavkm Looks like this should be focused on IPageHandlerModelConvention ... not IPagePropertyModelConvention.

[pranavkm]

@guardrex might be good to add a quip about property model \ parameter model conventions. Sort of along the lines of https://docs.microsoft.com/en-us/aspnet/core/mvc/controllers/application-model?view=aspnetcore-2.1#sample-modifying-the-parametermodel. For most parts, you could copy the docs \ samples verbatim.

[guardrex]

@pranavkm Where we stand today with this topic is that it focuses on conventions that implement IPageConvention ... so it's very RP options.Conventions-feature focused.

Given this issue is only to cover new 2.1 IPageConvention-based features (and there seems to be only one), let's address that ☝️ separately. #5958

[pranavkm]

Okies, sounds good.