develop: Add -b/--build-directory option to set build_directory package attribute (#39606)

* develop: Add -b/--build-directory option to set build_directory package attribute.

* Update docs

---------

Co-authored-by: psakievich <psakiev@sandia.gov>
Co-authored-by: vbrunini <vbrunini@users.noreply.github.com>

lib/spack/docs/environments.rst

@@ -416,6 +416,23 @@ that git clone if ``foo`` is in the environment.
 Further development on ``foo`` can be tested by reinstalling the environment,
 and eventually committed and pushed to the upstream git repo.
 
+If the package being developed supports out-of-source builds then users can use the
+``--build_directory`` flag to control the location and name of the build directory. 
+This is a shortcut to set the ``package_attributes:build_directory`` in the
+``packages`` configuration (see :ref:`assigning-package-attributes`).
+The supplied location will become the build-directory for that package in all future builds.
+
+.. warning::
+   Potential pitfalls of setting the build directory
+    Spack does not check for out-of-source build compatibility with the packages and
+    so the onerous of making sure the package supports out-of-source builds is on
+    the user.
+    For example, most ``autotool`` and ``makefile`` packages do not support out-of-source builds
+    while all ``CMake`` packages do.
+    Understanding these nuances are on the software developers and we strongly encourage
+    developers to only redirect the build directory if they understand their package's
+    build-system.
+
 ^^^^^^^
 Loading
 ^^^^^^^

lib/spack/docs/packages_yaml.rst

@@ -647,6 +647,8 @@ manually placed files within the install prefix are owned by the
 assigned group. If no group is assigned, Spack will allow the OS
 default behavior to go as expected.
 
+.. _assigning-package-attributes:
+
 ----------------------------
 Assigning Package Attributes
 ----------------------------