gopass insert [name] [key] fails to handle multiline YAML entries

[innovate-invent]

Summary

With a secret of the format:

pass
key1: |
  text
  moretext

inserting a new key will yield

pass
key1: |
key2: content
  text
  moretext

gopass insert also fails to properly handle inserting multiline YAML strings

even manually adding the entries, gopass show fails to output the content:

gopass show secret key1
key1: |

Steps To Reproduce

echo "|\n  text\n  moretext" | gopass insert secret key1
echo "content" | gopass insert secret key2

### Expected behavior
The secret should respect the YAML

pass
key1: |
text
moretext
key2: content

### Environment
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Please complete the following information (see note below)
-->

uname -a
Linux manjaro 5.11.19-1-MANJARO #1 SMP PREEMPT Fri May 7 17:34:25 UTC 2021 x86_64 GNU/Linux
gopass --version
gopass 1.12.6 (2021-05-11 15:27:04) go1.16.4 linux amd64

- Installation method: pacman

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**PLEASE NOTE**

There is a package named gopass in the official Debian repository.
This package is not related to this project in any way. If you
installed gopass from the Debian archives report any bugs in
the Debian BTS.
-->

### Additional context
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Add any other context about the problem here.
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[dominikschulz]

Hmm, I don't think I've ever tried inserting multi-line keys.
If you want to help fix this contributing a test case would be appreciated.

[innovate-invent]

I dug into the code and it appears to be a documentation issue:

gopass/pkg/gopass/secrets/yaml.go

Lines 21 to 32 in 1629395

	// YAML is a gopass secret that contains a parsed YAML data structure.
	// This is a legacy data type that is discouraged for new users as YAML
	// is neither trivial nor intuitive for users manually editing secrets (e.g.
	// unquoted phone numbers being parsed as octal and such).
	//
	// Format
	// ------
	// Line | Description
	// 0 | Password
	// 1-n | Body
	// n+1 | Separator ("---")
	// n+2 | YAML content

gopass/pkg/gopass/secrets/kv.go

Lines 38 to 72 in 1629395

	// KV is a secret that contains a password line (maybe empty), any number of
	// lines of key-value pairs (defined as: contains a colon) and any number of
	// free text lines. This is the default secret format gopass uses and encourages.
	// It should be compatible with most other password store implementations and
	// works well with our vanity features (e.g. accessing single entries in secret).
	//
	// Format
	// ------
	// Line | Description
	// ---- | -----------
	// 0 | Password. Must contain the "password" or be empty. Can not be omitted.
	// 1-n | Key-Value pairs, e.g. "key: value". Can be omitted but the secret
	// | might get parsed as a "Plain" secret if zero key-value pairs are found.
	// n+1 | Body. Can contain any number of characters that will be parsed as
	// | UTF-8 and appended to an internal string. Note: Technically this can
	// | be any kind of binary data but we neither support nor test this with
	// | non-text data. Also we do not intent do support any kind of streaming
	// | access, i.e. this is not intended for huge files.
	//
	// Example
	// -------
	// Line | Content
	// ---- | -------
	// 0 | foobar
	// 1 | hello: world
	// 2 | gopass: secret
	// 3 | Yo
	// 4 | Hi
	//
	// This would be parsed as a KV secret that contains:
	// - password: "foobar"
	// - key-value pairs:
	// - "hello": "world"
	// - "gopass": "secret"
	// - body: "Yo\nHi"

YAML format explicitly requires the "---" or it will be parsed as KV