The description of the "Format Specification Mini-Language" states for float and Decimal regarding presentation type 'f':
"The default precision is 6."
Regarding presentation type None it reads:
"Similar to 'g', except that fixed-point notation, when used, has at least one digit past the decimal point."
While both statements are accurate for float, they don't hold for Decimal.
In order to preserve the information about the decimal exponent, in both cases Decimal formatting displays as many fractional digits as dictated by it's exponent.

For a discussion of the different behaviour of float and Decimal see https://bugs.python.org/issue39077.

New changeset c642374b3ef72f6f300616f07aea2a3f9ed83e51 by Mark Dickinson in branch 'master':
bpo-39096: Improve description of 'e', 'f' and 'g' presentation types (#23537)
https://github.com/python/cpython/commit/c642374b3ef72f6f300616f07aea2a3f9ed83e51

New changeset cf47b3969e21f66655fcb414ff9bfdd503069c2d by Miss Islington (bot) in branch '3.9':
bpo-39096: Improve description of 'e', 'f' and 'g' presentation types (GH-23537) (GH-23550)
https://github.com/python/cpython/commit/cf47b3969e21f66655fcb414ff9bfdd503069c2d

New changeset c3009a5818112b5fb8867d786ce33e0e9489f4e0 by Miss Islington (bot) in branch '3.8':
bpo-39096: Improve description of 'e', 'f' and 'g' presentation types (GH-23537) (GH-23551)
https://github.com/python/cpython/commit/c3009a5818112b5fb8867d786ce33e0e9489f4e0

These changes are a big improvement. Thanks, Mark. Should this issue be closed, or are you thinking of more changes?

Personally I think we should leave it as it is now, and open another issue if people find fault with the new docs.

The 'default precision' issue is addressed; the issues that Michael reported about the `None` presentation type are still valid, I think.

> Should this issue be closed, or are you thinking of more changes?

I've made one more round of changes in GH-23575. Once that's gone in, I think we can close this issue.

I don't think we're ever going to be able to make this table perfectly accurate; at best, we can achieve a series of successive approximations to the truth. (Which is pretty much what I aim for when teaching.)

New changeset 886b2e5c7a2caf87070728dba8f18c3d65e51071 by Mark Dickinson in branch 'master':
bpo-39096: Format specification documentation fixes for numeric types (GH-23575)
https://github.com/python/cpython/commit/886b2e5c7a2caf87070728dba8f18c3d65e51071

New changeset 12032cdec5ae1e56d4e4b8206fb2e9cccc5a9f58 by Miss Islington (bot) in branch '3.8':
bpo-39096: Format specification documentation fixes for numeric types (GH-23575)
https://github.com/python/cpython/commit/12032cdec5ae1e56d4e4b8206fb2e9cccc5a9f58

New changeset b812e236df506603e592086269e088b00d021460 by Miss Islington (bot) in branch '3.9':
bpo-39096: Format specification documentation fixes for numeric types (GH-23575)
https://github.com/python/cpython/commit/b812e236df506603e592086269e088b00d021460