adds more info about valid docker ID

[sanscontext]

Closes #1104

Adds details about what makes a valid Docker ID, since this was missing from the validation on the field. (Also filed internal tickets to address this.)

Signed-off-by: LRubin lrubin@docker.com

[joaofnfernandes]

Feel free to merge this, but I think that details like these don't add much value to the docs:

• I'll not read the docs to learn how to create a Docker account. If my password needs to have at least 6 characters I'll get immediate feedback when creating the account
• These details will change as our security policies evolve, so not having this info in the docs will help the docs to be up to date