Improve GlSprintf documentation

What does this MR do?

This MR attempts to address #610 (closed). It:

1. places the Interpolated content examples first, since they're the more common/important use-case;
2. removes bare strings from examples which would not work in GitLab;
3. clarifies that any arbitrary HTML tags can also be used in interpolations;
4. clarifies how whitespace is preserved.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • Customers Portal: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.

Closes #610 (closed)