Update startCursor and endCursor in Connections.md spec (#2655)
Update startCursor and endCursor in Connections.md spec (#2655)

Summary:
Actual usage requires that `startCursor` and `endCursor` are nullable for empty results.

This is reflected in https://github.com/facebook/relay/issues/2122 and implemented in https://github.com/facebook/relay/commit/a17b462b3ff7355df4858a42ddda75f58c161302.

Pull Request resolved: https://github.com/facebook/relay/pull/2655

Test Plan:
Imported from GitHub, without a `Test Plan:` line.

**Static Docs Preview: relay**
|[Full Site](https://our.intern.facebook.com/intern/staticdocs/eph/D33685435/V3/relay/)|

|**Modified Pages**|

Reviewed By: kassens

Differential Revision: D33685435

Pulled By: alunyov

fbshipit-source-id: f02ffc07e88e5afbcc9eee58bf7d76435f4c43f4

voideanvalue/relay,kassens/relay,kassens/relay,xuorig/relay,josephsavona/relay,kassens/relay,xuorig/relay,voideanvalue/relay,wincent/relay,kassens/relay,facebook/relay,wincent/relay,wincent/relay,josephsavona/relay,wincent/relay,josephsavona/relay,xuorig/relay,wincent/relay,voideanvalue/relay,voideanvalue/relay,kassens/relay,xuorig/relay,xuorig/relay,facebook/relay,kassens/relay,facebook/relay,facebook/relay,xuorig/relay,wincent/relay,facebook/relay,voideanvalue/relay,voideanvalue/relay,facebook/relay