update docs (and examples) regarding PrevRealm and OrigCaller

[moul]

See https://github.com/gnolang/gno/pull/2918/files#r1791382196

[stefann-01]

Glad to see this issue raised, as I was about to create a similar one. I've provided more context related to the PR review you mentioned here: #2918 (comment)

Since I’ve already dug into this, I'd like to address the issue further by writing documentation and examples for these and other native.gno functions. I believe the best place for this would be under the Reference section of the documentation, specifically within docs/reference/stdlibs/std?