Cleanup installing docs

Author: eregon

Diff for: doc/user/installing-libssl.md

@@ -17,7 +17,7 @@ To compile TruffleRuby against a non-system `libssl`, set `OPENSSL_PREFIX` while
 export OPENSSL_PREFIX=/path/to/my/openssl-1.1.0
 ```
 
-### RedHat-based: Fedora, Oracle Linux, etc
+### Fedora-based: RHEL, Oracle Linux, etc
 
 ```bash
 sudo dnf install openssl-devel

Diff for: doc/user/installing-llvm.md

@@ -12,19 +12,19 @@ If you are using an older version, see [the documentation for that version](http
 
 The `make` utility as well as the dependencies of the toolchain still need to be available to build C and C++ extensions.
 
-## RedHat-based: Fedora, Oracle Linux, etc.
+### Fedora-based: RHEL, Oracle Linux, etc
 
 ```bash
 sudo dnf install make gcc
 ```
 
-## Debian-based: Ubuntu, etc.
+### Debian-based: Ubuntu, etc
 
 ```bash
 sudo apt-get install make gcc
 ```
 
-## Mandriva-based and other Linux distributions
+### Mandriva-based and other Linux distributions
 
 Note: Such distributions are not tested and not [supported](../../README.md#system-compatibility).
 
@@ -40,7 +40,7 @@ cd /usr/lib/gcc
 sudo ln -s x86_64-mandriva-linux-gnu x86_64-linux-gnu
 ```
 
-## macOS
+### macOS
 
 On macOS, make sure you have installed the command line developer tools from Xcode:
 

Diff for: doc/user/installing-zlib.md

@@ -8,7 +8,7 @@ permalink: /reference-manual/ruby/Installingzlib/
 
 TruffleRuby provides the `zlib` module but not the native `zlib` system library that the module uses.
 
-### RedHat-based: Fedora, Oracle Linux, etc
+### Fedora-based: RHEL, Oracle Linux, etc
 
 ```bash
 sudo dnf install zlib-devel

Diff for: doc/user/utf8-locale.md

@@ -18,7 +18,7 @@ locale
 shows no `="C"` and no warning.
 Instead, all values should be `"en_US.UTF-8"` or other regions but still `.UTF-8`.
 
-### RedHat-based: Fedora, Oracle Linux, etc
+### Fedora-based: RHEL, Oracle Linux, etc
 
 ```bash
 export LANG=en_US.UTF-8