docs: more info on currency digits

[cexbrayat]

Adds an example of using the currency pipe with a currency that has no cents like CLP,
which will format the amount with no digits if digitsInfo is not provided:

<!-- outputs CA$14.00 -->
{{ 14 | currency:'CAD' }}
<!-- outputs CLP14 -->
{{ 14 | currency:'CLP' }}

Amends the docs, adds an example and fix an error with a current example.

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

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
[x] Documentation content changes
[ ] angular.io application / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

See #24141 (comment)

What is the new behavior?

Adds a small example of using the currency pipe with no digitsInfo and with a currency that has no cents to illustrate that the formatting done by the pipe by default follows the ISO 4217 specification.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[jbogarthyde]

See also this pending PR on rounding behavior:
#24303

[brandonroberts]

@vicb will you provide an approval for this PR?

[mhevery]

http://test/OCL:204944974:BASE:204945010:1531852362437:7e2a8198

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