Avoid using HTML tags in translation strings (wp-admin/custom-header.php)

[ramiy]

See the attached patch.

• remove the <strong> tag from the string.
• add translators comments.
• split into smaller strings.
• use %s and %d instead of %1$d.

Generally, this patch creates 3 simpler strings without HTML tags, from 2 strings with HTML tags.

[ramiy]

Found two more strings! See the second patch. Same principle.

[ramiy]

Old strings:

• Images should be at least <strong>%1$d pixels</strong> wide.
• Images should be at least <strong>%1$d pixels</strong> tall.
• Suggested width is <strong>%1$d pixels</strong>.
• Suggested height is <strong>%1$d pixels</strong>.

New strings:

• Images should be at least %s wide.
• Images should be at least %s tall.
• Suggested width is %s.
• Suggested height is %s.
• %d pixels

As you can see the new string has no HTML tags, and they are easier to translate.

[SergeyBiryukov]

In 36658:

I18N: Remove <strong> tags from translatable strings in wp-admin/custom-header.php.

Add translator comments.

Props ramiy.
Fixes #35675.