Document HOMEBREW_BUILD_FROM_SOURCE

Additionally, add '--build-from-source' to the bash-completion script.

Library/Contributions/brew_bash_completion.sh

@@ -30,7 +30,7 @@ _brew_to_completion()
     if [[ ${COMP_WORDS[1]} == "install" && "$cur" == --* ]]; then
         local opts=$(
             local opts=(
-              --force --verbose --debug --use-clang --use-gcc --use-llvm --ignore-dependencies --HEAD
+              --force --verbose --debug --use-clang --use-gcc --use-llvm --ignore-dependencies --build-from-source --HEAD
               $(brew options --compact "$prev")
             )
             for o in ${opts[*]}; do

Library/Contributions/manpages/brew.1.md

@@ -290,6 +290,10 @@ Some sample commands ship with Homebrew and are enabled by default.
 
 ## ENVIRONMENT
 
+  * HOMEBREW\_BUILD\_FROM\_SOURCE:
+    If set, instructs Homebrew to compile from source even when a formula
+    provides a bottle.
+
   * HOMEBREW\_CACHE:
     If set, instructs Homebrew to use the give folder as the download cache.
     Otherwise, `~/Library/Caches/Homebrew` is used.

share/man/man1/brew.1

@@ -328,6 +328,10 @@ $ ls `brew \-\-repository`/Library/Contributions/examples
 .SH "ENVIRONMENT"
 .
 .TP
+HOMEBREW_BUILD_FROM_SOURCE
+If set, instructs Homebrew to compile from source even when a formula provides a bottle\.
+.
+.TP
 HOMEBREW_CACHE
 If set, instructs Homebrew to use the give folder as the download cache\. Otherwise, \fB~/Library/Caches/Homebrew\fR is used\.
 .