fix: force_implicit_tls config field name in docs

[Spaceman1701]

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Which user-facing changes does this PR introduce?

[BUGFIX] fix the name of `force_implicit_tls` in the configuration docs and add global `smtp_force_implicit_tls` to the docs. Both fields already exist, but the documentation was incorrect.

#4818 added force_implicit_tls to the email receiver, but accidentally documented the field as implicit_tls. The documentation in that change also forgot to mention the smpt_force_implicit_tls filed in the the global section. This PR just modifies the docs to match the existing behavior.

See

alertmanager/config/notifiers.go

Line 326 in 07e5b92

ForceImplicitTLS *bool `yaml:"force_implicit_tls,omitempty" json:"force_implicit_tls,omitempty"`

and

alertmanager/config/config.go

Line 816 in 07e5b92

SMTPForceImplicitTLS *bool `yaml:"smtp_force_implicit_tls,omitempty" json:"smtp_force_implicit_tls,omitempty"`

Thanks @nutmos for discovering this!

[SoloJacobs]

Ah, a classic 😃

[Spaceman1701]

I think one review is enough to go ahead and merge. Doing that now :)