docs: document that an empty env var falls back to envDefault

[Pastalikek65]

Fixes #428.

envDefault is documented as "sets the default value for the field", but a
variable set to an empty value also gets the default (behavior introduced in
#248, v7.0.0).

Adds:

• a note in the Caveats section
• a pointer on the envDefault bullet
• a runnable example, covered by go test and shown on pkg.go.dev

No behavior changes.

[Pastalikek65]

Friendly bump — the docs fix is ready for review; CI is waiting on workflow approval for first-time contributors. Thanks!

[caarlos0]

thanks!