I18N string improvements for 7.0

[desrosj]

This ticket is for any string improvements that do not directly relate to specific tickets.

Previously:

• #63629 (6.9)

### Summary
This PR addresses several I18N string improvements as part of the WordPress 7.0 release cycle.

### Changes

• Added missing translator comments to several user-facing error messages that lacked context for translators. These include SMTP-related error strings and informational messages containing technical terms (PHP function names, protocol extension names, and email standard names) that translators need to know should not be translated.
• Fixed a misplaced translator comment that was positioned too far from its associated translatable string (separated by a conditional block), causing translation tooling such as WP-CLI i18n make-pot and Poedit to fail to associate the comment with the string.
• Fixed inconsistent capitalization in a user-facing warning string where the first word after the label prefix was lowercase, inconsistent with WordPress core style.

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Trac ticket

• https://core.trac.wordpress.org/ticket/64244

## Use of AI Tools

[SergeyBiryukov]

In 62030:

I18N: Correct translator comment placement in wp_notify_postauthor().

Follow-up to [61179].

Props sanketparmar, mukesh27.
See #64244.

[desrosj]

Opened #64899 for the 7.1 release cycle. With 7.0 RC1 due out today, going to close this out. This can be reopened or referenced if there are more changes needed related to this during 7.0.

Thanks for the PR! The misplaced comment in wp_notify_postauthor() was fixed in r62030.

The rest of the strings don't seem to have any placeholders or trailing spaces, so adding comments for them might be redundant, as noted above.