docs(elements): move clarification about custom elements

[BBKolton]

Moved clarification that "custom elements" are a subtype of Web Components to the first use of the term "custom elements"

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• angular.io application / infrastructure changes
• Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

No code changes

Does this PR introduce a breaking change?

• Yes
• No

[gkalpak]

👍
Can you please change the commit message to something like:

docs(elements): move clarification about custom elements

...

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[BBKolton]

Updated the commit message, forgive me if this isn't the format you're looking for. First contribution to open source (even if small) :)

[BBKolton]

Proc for cla/google bot

[BBKolton]

proc cla/google bot 2: electric boogaloo

[googlebot]

CLAs look good, thanks!

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