[DOCS]Add a lower-casing eamil address example to Keyword Tokenizer

[jureaky]

While reading a document for Keyword Tokenizer, I felt a bit confused why Keyword Tokenizer is needed.

Keyword Tokenizer is too simple to explain, which makes an example also simple.
However, as the document stated the other use-case(lower-casing email address) for users
to better understand the necessity of Keyword Tokenizer, I added the corresponding example.

This is AS-IS:

This is Added Example:

[elasticmachine]

Pinging @elastic/es-search (:Search/Analysis)

[elasticmachine]

Pinging @elastic/es-docs (>docs)

[jrodewig]

Changing the spacing to be more consistent with our other examples.

Let's also use example.com as the placeholder domain.

Suggested change

	"filter": ["lowercase"],
	"text": "john.SMITH@global-international.COM"
	"filter": [ "lowercase" ],
	"text": "john.SMITH@example.COM"

[jrodewig]

Suggested change

	"token": "john.smith@global-international.com",
	"start_offset": 0,
	"end_offset": 35,
	"token": "john.smith@example.com",
	"start_offset": 0,
	"end_offset": 22,

[jrodewig]

Suggested change

	[ john.smith@global-inetrnational.com ]
	[ john.smith@example.com ]

[jrodewig]

Suggested change

	The above sentence would produce the following term:
	The request produces the following token:

[jrodewig]

Thanks for the PR @jureaky.

The snippet looks good. I left some suggestions to fix typos and add some text to provide context.

I'd like to take another look after you've had a chance to go through those.

[jrodewig]

@elasticmachine test this please

[jrodewig]

Looks like GIthub ate my original suggestion. I'd add some lead-in text here to provide context. The original heading contained a typo.

Suggested change

	[float]
	=== Exmaple output(Lower-casing email address)
	[discrete]
	[[analysis-keyword-tokenizer-token-filters]]
	=== Combine with token filters
	You can combine the `keyword` tokenizer with token filters to normalise
	structured data, such as product IDs or email addresses.
	For example, the following <<indices-analyze,analyze API>> request uses
	`keyword` tokenizer and <<analysis-lowercase-tokenfilter,`lowercase`>> filter to
	convert an email address to lowercase.

[jureaky]

@jrodewig I changed all you suggested and force-pushed after rebase... PTAL !

[jrodewig]

LGTM. Thanks @jureaky. I'll get this merged and backported.

[jrodewig]

@elasticmachine test this please

[jrodewig]

Backport commits

master 4fe8ad3
7.x 21f362a
7.7 0813ec4
7.6 8983bc3