Matching forward declaration of functions and classes

[josh11b]

Description of the bug:

• Question-for-leads issue Open question: Calling functions defined later in the same file #472 and proposal Principle: information accumulation #875 established that Carbon will use forward declarations.
• Question-for-leads issue How do we match forward declarations with their definitions? #1132 described the general rules for how to match forward declarations with their definitions.
• Proposal Generics details 9: forward declarations #1084 implemented the decision in How do we match forward declarations with their definitions? #1132 to allow forward declaration of interfaces and implementations.
• Proposal Nominal classes and methods #722 and PR Clarify class declaration syntax #1026 added support for forward declaring classes
• PR Updating function and variable docs #1017 explained how to forward declare functions

What is missing is an update to the description of function and class declarations to include the matching rules from #1132. In particular, that they are matched by name and if a declaration and definition have matching names but are different in other details, then an error should be reported at compile time.

It isn't super clear to me if making these changes should be a proposal or just a PR.

Link to documentation:

• forward declaration of classes
• forward declaration of functions
• Declarations, Definitions, and Scopes in the overview