Update the design docs for functions to reflect the lambdas proposal

[geoffromer]

#3848 introduced some changes to the design of named functions, as part of generalizing them to encompass lambda expressions. For example:

• They can take positional parameters instead of named parameters.
• They can be defined with => expr; instead of -> ReturnType { body }
• They can have captures.

Lambda expressions themselves were added to the design documentation in #5300, but docs/design/functions.md and docs/design/README.md should also be updated to reflect the changes to named functions.

[IvanDuran02]

should we move lambdas in table of contents (currently under "Unfinished tales") to functions?

Example:

• Functions
  • Parameters
  • auto return type
  • Block and statements
  • Lambdas <---- Example
  • Control Flow
    • if and else
    • etc...

This would also mean the short summary for lambdas in the docs/design/README.md should be done too?

[geoffromer]

That seems like a good idea. I don't think it necessarily needs to be done to resolve this issue, but if it's convenient to do that along the way, great!