Document webpack change: improve merging of resolve and parsing options

[webpack-bot]

A pull request by @sokra was merged and maintainers requested a documentation change.

See pull request: webpack/webpack#9422

---

by rules and via loader API

Arrays overwrite by default, but can reference old value with "..."

partial backport of webpack/webpack#9126

What kind of change does this PR introduce?
feature

Did you add tests for your changes?
yes

Does this PR introduce a breaking change?
no

What needs to be documented once your changes are merged?
Merging of Rule.resolve/Rule.parser objects has a new feature:
In array the special "..." item can now be used to reference to old value, while otherwise array values overwrite each other.
This also affects the getResolve(options) API in the loader API

[EugeneHlushko]

Wow this is very useful, we had also missed one feature in docs. Wonder if it was merged before the doc Issue on PR merge feature was introduced.

[EugeneHlushko]

from #3199:
Although to completely close the issue we need to figure our where to add the documentation on the getResolve resolver method. Currently its not on webpack.js.org and not on https://github.com/webpack/enhanced-resolve