Revise UI text for Kroki settings

What does this MR do?

Revises UI text per #330191 (closed). Also fixes some split sentences per the the guidelines in our externalization docs: https://docs.gitlab.com/ee/development/i18n/externalization.html#splitting-sentences

Screenshots or Screencasts (strongly suggested)

Before

After

• Updated list of supported formats to match docs
• Added 'Learn more' link to docs
• Shortened helper text throughout (a lot of additional info is already available in docs)
• Added a serial comma to the BlockDiag options

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Related to #330191 (closed)