docs: Update DashboardMetadata docs

[Spacerat]

I noticed that the DashboardMetadata docs are outdated, and reference e.g. 'lastreloadtime' which no longer exists, and is instead implemented in a separate model.

This updates the docs to reflect the current state of Dashboard metadata.

[feng-tao]

flake8 .
/opt/hostedtoolcache/Python/3.7.9/x64/lib/python3.7/site-packages/pycodestyle.py:113: FutureWarning: Possible nested set at position 1
  EXTRANEOUS_WHITESPACE_REGEX = re.compile(r'[[({] | []}),;:]')
./databuilder/models/dashboard/dashboard_metadata.py:23: [W293] blank line contains whitespace

a flake8 error

[feng-tao]

thanks @Spacerat !

[Spacerat]

for sure! and thanks for fixing the lint error :)