Support multi-line strings delimiters for documentation metadata

[yannham]

Is your feature request related to a problem? Please describe.
Currently, specifying a documentation on a field is done by writing a static string delimited by double quotes. This becomes annoying when the documentation contains code and examples that themselves involve strings, since one then has to escape quotes. The semantics is not really clear at first sight:

item | doc "
         # Usage

         Example of usage: `let str = \"some str\" in item str`
       "
[...]

Describe the solution you'd like
Support the same set of delimiters as for normal strings, such as variable length delimiters, for documentation as well:

item | doc m#"
         # Usage

         Example of usage: `let str = "some str" in item str`
       "#m
[...]

Describe alternatives you've considered
Using different delimiters for documentation, like several consecutive (double) quotes, as in say Python docstrings. However, the issue of not clashing with delimiters is exactly the same as for normal strings, and we've already solved it, so it doesn't really make sense to do something different here.