docs: validation-set documentation is missing snap components

[jocado]

Snap component specification is supported in a validation-set, alongside the snap for which the component is for.
This should be reflected in the documentation.

Please check, but I believe the format support is as follows:

type:            validation-set
authority-id:     <authority account id>
series:         16
account-id:     <account-id>
name:            <validation set name>
revision:         <int>
sequence:         <int>
snaps:
  - name:      <snap-name>
    id:        <snap id>
    presence: "required"|"optional"|"invalid" # optional
                                             # defaults to required
    revision: <n> # the revision of the snap. optional
    components:   # optional
       <component-name-1>: "optional"|"required"|"invalid" # presence
       <component-name-2>:
           presence: "optional"|"required"|"invalid"           
           revision: <n> # optional
      
 ...
timestamp:    <utc-datetime>

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Reported from: https://documentation.ubuntu.com/core/reference/assertions/validation-set/