[PATCH] doc: document PERL_INLINE_DIRECTORY usage

[Eric Wong <e@80x24.org>]

For now, we will document this since it allows better
performance without the burden of extensions.  Perhaps one day
far in the future Perl can natively support vfork(2) AND that
version of Perl will be widely available, but I suspect that day
is at least a decade away, if not two:

	https://rt.perl.org/Ticket/Display.html?id=128227
---
 Documentation/public-inbox-daemon.pod | 12 ++++++++++++
 Documentation/public-inbox-watch.pod  |  6 ++++++
 2 files changed, 18 insertions(+)

diff --git a/Documentation/public-inbox-daemon.pod b/Documentation/public-inbox-daemon.pod
index 42beda6..72794a5 100644
--- a/Documentation/public-inbox-daemon.pod
+++ b/Documentation/public-inbox-daemon.pod
@@ -133,6 +133,18 @@ See L<public-inbox-config(5)>
 Used by systemd (and compatible) installations for socket
 activation.  See L<systemd.socket(5)> and L<sd_listen_fds(3)>.
 
+=item PERL_INLINE_DIRECTORY
+
+Pointing this to point to a writable directory enables the use
+of L<Inline> and L<Inline::C> extensions which may provide
+platform-specific performance improvements.  Currently, this
+enables the use of L<vfork(2)> which speeds up subprocess
+spawning with the Linux kernel.
+
+public-inbox will never enable L<Inline::C> automatically without
+this environment variable set.  See L<Inline> and L<Inline::C>
+for more details.
+
 =back
 
 =head1 UPGRADING
diff --git a/Documentation/public-inbox-watch.pod b/Documentation/public-inbox-watch.pod
index 404303e..a59ba32 100644
--- a/Documentation/public-inbox-watch.pod
+++ b/Documentation/public-inbox-watch.pod
@@ -101,6 +101,12 @@ startup.
 config file. default: ~/.public-inbox/config
 See L<public-inbox-config(5)>
 
+=item PERL_INLINE_DIRECTORY
+
+This may affect any public-inbox processes, but is intended
+for long-lived ones such as C<public-inbox-watch> or network
+daemons.  See L<public-inbox-daemon(8)>.
+
 =back
 
 =head1 CONTACT
-- 
EW