stale-symlink-buildworld.sh: improve documentation

stale symlinks: configurable obj dir prefix

Use $MAKEOBJDIRPREFIX if set, otherwise /usr/obj

stale-symlink-buildworld.sh: a script to check for stale symlinks on a FreeBSD system

You can run the script before or after `make installworld'

You may also check your local ports with:

env STA

stale-symlink-buildworld.sh: a script to check for stale symlinks on a FreeBSD system

You can run the script before or after `make installworld'

You may also check your local ports with:

env STALE_SYMLINK_BUILDWORLD_DIRS=/usr/local ./stale-symlink-buildworld.sh

PR: 276235

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