doc: Document detect_releasevers and update example

Adds dnf.rpm.detect_releasevers to the API docs and mention it is
now preferred over dnf.rpm.detect_releasever.

Updates examples/install_extension.py to use detect_releasevers and set
the releasever_major and releasever_minor substitution variables.

Author: evan-goode

doc/api_rpm.rst

@@ -27,6 +27,14 @@
 
   Returns ``None`` if the information can not be determined (perhaps because the tree has no RPMDB).
 
+.. function:: detect_releasevers(installroot)
+
+  Returns a tuple of the release name, overridden major release, and overridden minor release of the distribution of the tree rooted at `installroot`. The function uses information from RPMDB found under the tree. The major and minor release versions are usually derived from the release version by splitting it on the first ``.``, but distributions can override the derived major and minor versions. It's preferred to use ``detect_releasevers`` over ``detect_releasever``; if you use the latter, you will not be aware of distribution overrides for the major and minor release versions.
+
+  Returns ``(None, None, None)`` if the information can not be determined (perhaps because the tree has no RPMDB).
+
+  If the distribution does not override the release major version, then the second item of the returned tuple will be ``None``. Likewise, if the release minor version is not overridden, the third return value will be ``None``.
+
 .. function:: basearch(arch)
 
   Return base architecture of the processor based on `arch` type given. E.g. when `arch` i686 is given then the returned value will be i386.

doc/examples/install_extension.py

@@ -32,8 +32,12 @@
 
     with dnf.Base() as base:
         # Substitutions are needed for correct interpretation of repo files.
-        RELEASEVER = dnf.rpm.detect_releasever(base.conf.installroot)
+        RELEASEVER, MAJOR, MINOR = dnf.rpm.detect_releasever(base.conf.installroot)
         base.conf.substitutions['releasever'] = RELEASEVER
+        if MAJOR is not None:
+            base.conf.substitutions['releasever_major'] = MAJOR
+        if MINOR is not None:
+            base.conf.substitutions['releasever_minor'] = MINOR
         # Repositories are needed if we want to install anything.
         base.read_all_repos()
         # A sack is required by marking methods and dependency resolving.