Document HOMEBREW_DOWNLOAD_CONCURRENCY environment variable

We're going to be ready in 4.6.0 for users to try this out (but not
yet for it to be the default).

Author: MikeMcQuaid

Library/Homebrew/env_config.rb

@@ -182,6 +182,13 @@ module EnvConfig
         description: "Use this bearer token for authenticating with a Docker registry proxying GitHub Packages. " \
                      "Preferred over `$HOMEBREW_DOCKER_REGISTRY_BASIC_AUTH_TOKEN`.",
       },
+      HOMEBREW_DOWNLOAD_CONCURRENCY:             {
+        description: "If set, Homebrew will download in parallel using this many concurrent connections. " \
+                     "Setting to `auto` will use twice the number of available CPU cores " \
+                     "(what our benchmarks showed to produce the best performance). " \
+                     "If set to `1` (the default), Homebrew will download in serial.",
+        default:     1,
+      },
       HOMEBREW_EDITOR:                           {
         description:  "Use this editor when editing a single formula, or several formulae in the " \
                       "same directory." \
@@ -633,7 +640,6 @@ def devcmdrun?
 
     sig { returns(Integer) }
     def download_concurrency
-      # TODO: document this variable when ready to publicly announce it.
       concurrency = ENV.fetch("HOMEBREW_DOWNLOAD_CONCURRENCY", 1)
       concurrency = if concurrency == "auto"
         require "os"

docs/Manpage.md

@@ -4043,6 +4043,15 @@ command execution e.g. `$(cat file)`.
 : Use this bearer token for authenticating with a Docker registry proxying
   GitHub Packages. Preferred over `$HOMEBREW_DOCKER_REGISTRY_BASIC_AUTH_TOKEN`.
 
+`HOMEBREW_DOWNLOAD_CONCURRENCY`
+
+: If set, Homebrew will download in parallel using this many concurrent
+  connections. Setting to `auto` will use twice the number of available CPU
+  cores (what our benchmarks showed to produce the best performance). If set to
+  `1` (the default), Homebrew will download in serial.
+  
+  *Default:* `1`.
+
 `HOMEBREW_EDITOR`
 
 : Use this editor when editing a single formula, or several formulae in the same

manpages/brew.1

@@ -2617,6 +2617,13 @@ Use this base64 encoded username and password for authenticating with a Docker r
 \fBHOMEBREW_DOCKER_REGISTRY_TOKEN\fP
 Use this bearer token for authenticating with a Docker registry proxying GitHub Packages\. Preferred over \fB$HOMEBREW_DOCKER_REGISTRY_BASIC_AUTH_TOKEN\fP\&\.
 .TP
+\fBHOMEBREW_DOWNLOAD_CONCURRENCY\fP
+If set, Homebrew will download in parallel using this many concurrent connections\. Setting to \fBauto\fP will use twice the number of available CPU cores (what our benchmarks showed to produce the best performance)\. If set to \fB1\fP (the default), Homebrew will download in serial\.
+.RS
+.P
+\fIDefault:\fP \fB1\fP\&\.
+.RE
+.TP
 \fBHOMEBREW_EDITOR\fP
 Use this editor when editing a single formula, or several formulae in the same directory\.
 .RS