[Rd] Consider exporting some of the .Rd_get_xxx functions in tools

[Tim Taylor]

Would R-core consider exporting some of the .Rd_get_ family of functions 
within tools (e.g. tools:::.Rd_get_section(), tools:::.Rd_get_metadata())?

Whilst these are currently internal there is some use of them within 
documentation (e.g. help("Rd_db") uses tools:::.Rd_get_metadata() in 
it's examples) which hopefully gives some justification for exporting.

Best

Tim