doc: shell option for the execFile and execFileSync functions

[jvelezpo]

Useful for executing in a shell because it accepts arguments as
an array instead of a string as exec does.
Depending on the circumstances,
that can prove to be useful if the arguments are already prepared.

Fixes: #18199

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

doc