ARROW-5442: [Website] Clarify what makes a release artifact "official"

Author: Neal Richardson <neal.p.richardson@gmail.com>

Closes #4411 from nealrichardson/official-builds and squashes the following commits:

95a6262a6 <Neal Richardson> Remove one more 'official'
c96ef34bd <Neal Richardson> Explain 'official' releases and reorg a little
diff --git a/install.md b/install.md
index 06959c4..357b15ea 100644
--- a/install.md
+++ b/install.md
@@ -26,7 +26,7 @@
 
 See the [release notes][10] for more about what's new.
 
-### Source release
+### Source Release
 
 * **Source Release**: [{{site.data.versions['current'].tarball_name}}][6]
 * **Verification**: [asc signature][13], [sha256 checksum][14], [sha512 checksum][15], ([verification instructions][12])
@@ -37,25 +37,7 @@
 
 [Java Artifacts on Maven Central][4]
 
-## Binary Installers for C, C++, Python
-
-### C++ and Python Conda Packages (Unofficial)
-
-We have provided binary conda packages on [conda-forge][5] for the following
-platforms:
-
-* Linux and macOS (Python 2.7, 3.6 and 3.7)
-* Windows (Python 3.6 and 3.7)
-
-Install them with:
-
-
-```shell
-conda install arrow-cpp={{site.data.versions['current'].pinned_number}} -c conda-forge
-conda install pyarrow={{site.data.versions['current'].pinned_number}} -c conda-forge
-```
-
-### Python Wheels on PyPI
+### Python Wheels
 
 We have provided official binary wheels on PyPI for Linux, macOS, and Windows:
 
@@ -69,7 +51,26 @@
 These include the Apache Arrow and Apache Parquet C++ binary libraries bundled
 with the wheel.
 
-### C++ and GLib (C) Packages on Homebrew (Unofficial)
+## Other Binary Installers
+
+For convenience, we also provide binaries through several package managers, built from the source release. As the Apache Arrow PMC has not explicitly voted on these packages, they are technically considered unofficial releases.
+
+### C++ and Python Conda Packages
+
+Binary conda packages are on [conda-forge][5] for the following
+platforms:
+
+* Linux and macOS (Python 2.7, 3.6 and 3.7)
+* Windows (Python 3.6 and 3.7)
+
+Install them with:
+
+```shell
+conda install arrow-cpp={{site.data.versions['current'].pinned_number}} -c conda-forge
+conda install pyarrow={{site.data.versions['current'].pinned_number}} -c conda-forge
+```
+
+### C++ and GLib (C) Packages on Homebrew
 
 On macOS, you can install the C++ library using
 [Homebrew][17]:
@@ -84,7 +85,7 @@
 brew install apache-arrow-glib
 ```
 
-### C++ and GLib (C) Packages for Debian GNU/Linux, Ubuntu and CentOS (Unofficial)
+### C++ and GLib (C) Packages for Debian GNU/Linux, Ubuntu and CentOS
 
 We have provided APT and Yum repositories for Apache Arrow C++ and
 Apache Arrow GLib (C). Here are supported platforms:
@@ -210,9 +211,9 @@
 sudo yum install -y --enablerepo=epel parquet-glib-devel # For Parquet GLib (C)
 ```
 
-### C++ and GLib (C) Packages for MSYS2 (Unofficial)
+### C++ and GLib (C) Packages for MSYS2
 
-The official MSYS2 packages include [Apache Arrow C++ and GLib (C)
+The MSYS2 packages include [Apache Arrow C++ and GLib (C)
 package][16]. You can install the package by `pacman`.
 
 64bit version: