Docs: promote the shelve module in the dbm docs introduction

[andresdelfino]

I believe the proposed modification lets the user know earlier if the module is useful to them and that shelve may help him/her if the dbm module is not enough.

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📚 Documentation preview 📚: https://cpython-previews--126218.org.readthedocs.build/

[serhiy-storchaka]

@rhettinger, do you like to look at this?

[erlend-aasland]

There is already a .. seealso that points to shelve. If we are to make any changes, consider instead to make that reference more prominent, for example by moving it up to the introduction. IMO, that should be sufficient.

[andresdelfino]

There is already a .. seealso that points to shelve. If we are to make any changes, consider instead to make that reference more prominent, for example by moving it up to the introduction. IMO, that should be sufficient.

Hmmm, the PR does also make that change. Perhaps I'm not understanding you correctly?

[erlend-aasland]

Hmmm, the PR does also make that change. Perhaps I'm not understanding you correctly?

Sorry, my bad; I misread the PR to just add a new .. seealso. Yes, this particular change is fine with me.

I think some amendments are needed