Warnings during doc build: missing event-log-object

[veox]

• Version: v5.0.0-alpha.9-37-gf4a7d8c5 (current master, commit f4a7d8c)
• Python: 3.7.3
• OS: Arch Linux
• pip freeze output: MISSING

What was wrong?

During make docs:

/home/veox/src/web3.py/docs/contracts.rst:714: WARNING: undefined label: event-log-object (if the link has no caption the label must precede a section header)
/home/veox/src/web3.py/docs/releases.rst:671: WARNING: undefined label: event-log-object (if the link has no caption the label must precede a section header)

Direct fallout of PR #1032.

How can it be fixed?

• Review docs.
• Build docs in CI and fail on warnings.
• "Lint" docs.