chore: add synthetics url field description

[mderemer-nr]

Please follow conventional commit standards
in your commit messages and pull request title.

Give us some context

• What problems does this PR solve?
  • Links to URL definition and specifies we only allow http and https schemes
• Add any context that will help us review your changes such as testing notes,
  links to related docs, screenshots, etc.
• If your issue relates to an existing GitHub issue, please link to it.
  • It's better to describe what is supported url syntax by Synthetics monitor #3928

[CLAassistant]

All committers have signed the CLA.

[github-actions]

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[mderemer-nr]

Had to redo this in order to sign cla

[mderemer-nr]

@urbiz-nr This is a dup of the previous one i closed. Had to switch users in order to accept cla.

[zuluecho9]

@mderemer-nr I made a light edit to it; didn't think we needed to link to definition of URL but curious if you disagree. Thank you!

[mderemer-nr]

@zuluecho9 Thats fine by me.

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