document command line flag --project

[tpapp]

Eg in julia --help, and also the manual. At least that the flag exists and sets the project, the concept of the project is (will be?) explained in the relevant section of the manual (maybe cross-reference).

cf 955a950, #27816

[ihnorton]

Manual doc added in #28556

[tpapp]

I think that #28556 primarily documents JULIA_PROJECT, and mentions --project in passing. Documenting --project in --help and the manual would still be useful.