Post-fix doc tag names to be more descriptive.

Change doc tag names on stage changes to be more descriptive.
And add missing copyright statements  on changes.

doc/src/reference.adoc

@@ -237,7 +237,8 @@ include::../../src/tools/features/dependency-feature.jam[tag=impl-doc]
 include::../../src/tools/features/force-include-feature.jam[tag=doc]
 include::../../src/tools/features/include-feature.jam[tag=doc]
 include::../../src/tools/features/optimization-feature.jam[tag=inline-doc]
-include::../../src/tools/stage.jam[tag=features-doc]
+include::../../src/tools/stage.jam[tag=features-install-package-doc]
+include::../../src/tools/stage.jam[tag=features-install-prefix-doc]
 include::../../src/tools/features/instruction-set-feature.jam[tag=doc]
 include::../../src/tools/features/library-feature.jam[tag=doc]
 include::../../src/tools/features/find-lib-feature.jam[tag=path-doc]
@@ -256,7 +257,7 @@ include::../../src/tools/features/rtti-feature.jam[tag=doc]
 include::../../src/tools/features/runtime-feature.jam[tag=doc]
 include::../../src/tools/features/search-feature.jam[tag=doc]
 include::../../src/tools/features/source-feature.jam[tag=doc]
-include::../../src/tools/stage.jam[tag=features2-doc]
+include::../../src/tools/stage.jam[tag=features-staging-prefix-doc]
 include::../../src/tools/features/stdlib-feature.jam[tag=doc]
 include::../../src/tools/features/strip-feature.jam[tag=doc]
 include::../../src/tools/features/dll-feature.jam[tag=suppress-doc]

example/named-install-dirs/build.jam

@@ -47,3 +47,8 @@ install (datarootdir)/a9 : a : <staging-prefix>q/r/s ;
 
 
 build-project x ;
+
+# Copyright 2020 Dmitry Arkhipov
+# Distributed under the Boost Software License, Version 1.0.
+# (See accompanying file LICENSE_1_0.txt or copy at
+# http://www.boost.org/LICENSE_1_0.txt)

example/named-install-dirs/x/y/build.jam

@@ -2,3 +2,8 @@
 # from its parent
 
 install (docdir)/y1 : ../../a ;
+
+# Copyright 2020 Dmitry Arkhipov
+# Distributed under the Boost Software License, Version 1.0.
+# (See accompanying file LICENSE_1_0.txt or copy at
+# http://www.boost.org/LICENSE_1_0.txt)

example/named-install-dirs/x/z/build.jam

@@ -4,3 +4,8 @@
 project subz ;
 
 install (docdir)/z1 : ../../a ;
+
+# Copyright 2020 Dmitry Arkhipov
+# Distributed under the Boost Software License, Version 1.0.
+# (See accompanying file LICENSE_1_0.txt or copy at
+# http://www.boost.org/LICENSE_1_0.txt)

src/tools/stage.jam

@@ -1,6 +1,7 @@
 # Copyright 2003 Dave Abrahams
 # Copyright 2005, 2006 Rene Rivera
 # Copyright 2002, 2003, 2004, 2005, 2006 Vladimir Prus
+# Copyright 2020 Dmitry Arkhipov
 # Distributed under the Boost Software License, Version 1.0.
 # (See accompanying file LICENSE_1_0.txt or copy at
 # http://www.boost.org/LICENSE_1_0.txt)
@@ -37,13 +38,13 @@ feature.feature <so-version>           :        : free incidental ;
 feature.feature <install-no-version-symlinks> : on : optional incidental ;
 
 
-#| tag::features-doc[]
+#| tag::features-install-package-doc[]
 
 [[bbv2.builtin.features.install-package]]`install-package`::
 Specifies the name of the package to which installed files belong.  This is
 used for default installation prefix on certain platforms.
 
-|# # end::features-doc[]
+|# # end::features-install-package-doc[]
 
 feature.feature install-package : : free ;
 
@@ -109,7 +110,7 @@ rule install-dir-names ( )
 }
 
 
-#| tag::features-doc[]
+#| tag::features-install-prefix-doc[]
 
 [[bbv2.builtin.features.install-prefix]]`install-<name>`::
 Specifies installation prefix for <<bbv2.tasks.installing,`install`>> targets.
@@ -143,7 +144,7 @@ These named installation prefixes are registered by default:
 If more are necessary, they could be added with
 <<bbv2.reference.modules.stage.add-install-dir,`stage.add-install-dir`>>.
 
-|# # end::features-doc[]
+|# # end::features-install-prefix-doc[]
 
 feature.feature install-prefix : : free path ;
 add-install-dir exec-prefix : "" : prefix ;
@@ -170,7 +171,7 @@ add-install-dir localedir : locale : datarootdir ;
 add-install-dir mandir : man : datarootdir ;
 
 
-#| tag::features2-doc[]
+#| tag::features-staging-prefix-doc[]
 
 [[bbv2.builtin.features.staging-prefix]]`staging-prefix`::
 Specifies staging prefix for <<bbv2.tasks.installing,`install`>> targets.
@@ -188,7 +189,7 @@ into their intented locations during the build (such as when cross-compiling),
 but still need to communicate those intended locations to the build system,
 e.g. to generate configuration files.
 
-|# # end::features2-doc[]
+|# # end::features-staging-prefix-doc[]
 
 feature.feature staging-prefix : : free path ;