Use stripping block (`|-`) for page descriptions (#14980)
## Summary
Resolves #14976.
Currently, we uses this "[plain
scalar](https://yaml.org/spec/1.2.2/#733-plain-style)" format:
```yaml
description: Checks for `if key in dictionary: del dictionary[key]`.
```
Plain scalar must not contain the sequence `: `, however, so the above
is invalid.
This PR changes that to:
```yaml
description: |-
Checks for `if key in dictionary: del dictionary[key]`.
```
`|` denotes a "[block
scalar](https://yaml.org/spec/1.2.2/#81-block-scalar-styles)", whereas
[the `-` chomping
indicator](https://yaml.org/spec/1.2.2/#8112-block-chomping-indicator)
requires that a trailing newline, if any, must be stripped.
## Test Plan
