Remove spy scenario that is wrong level of detail for relish docs.

Behaviour is already covered by specs.
Merge pull request #821 from rspec/remove-spy-docs

Remove spy scenario that is wrong level of detail for relish docs.
Fix verified doubles when their const has already been stubbed.

Fixes #823.
Remove unnecessary captured block.

Capturing blocks isn’t free, and `yield` (which
we are already using) performs better than
capturing the block and using `block.call`.
Fix memoization to properly memoize nil.
Merge pull request #824 from rspec/fix-instance_double-when-class-const-stubbed

Fix verified doubles when their const has already been stubbed.
Badges are crisper as svg

[skip ci]
Merge pull request #827 from rspec/badges_as_s

Switch readme badges to svg
Validate invocation args for verified null object doubles.
Merge pull request #829 from rspec/fix-null-object-verified-double-arg-verification

Validate invocation args for verified null object doubles.
Add a `description` to `receive` matcher.
Add a description for `receive_message_chain` matcher.
Add a description for `receive_messages` matcher.
Add changelog entry.
Merge pull request #828 from rspec/add-description-for-receive-matcher

Add description for matchers
allow verifying doubles to use custom names in exceptions
add class documentation about named verifying doubles
mention naming verifying doubles in README
Merge pull request #826 from e2/issue_825_named_verifying_doubles

allow naming verifying doubles just like ordinary doubles
Add changelog entry for #826.

[ci skip]