[PATCH] INSTALL: note about make man with Asciidoctor backend

[Bagas Sanjaya <bagasdotme@gmail.com>]

"make man" can now be also done with Asciidoctor's manpage backend
instead of asciidoc+xmlto.

Update INSTALL to reflect that.

Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
---

 This patch is based on  "doc: add an option to have Asciidoctor build
 man pages directly" series by brian m. carlson [1]. It can be added
 to that series.

 [1]:
https://lore.kernel.org/git/20210512021138.63598-1-sandals@crustytoothpaste.net/

 INSTALL | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/INSTALL b/INSTALL
index 66389ce059..89e31566c3 100644
--- a/INSTALL
+++ b/INSTALL
@@ -184,8 +184,9 @@ Issues of note:
 
    "make doc" builds documentation in man and html formats; there are
    also "make man", "make html" and "make info". Note that "make html"
-   requires asciidoc, but not xmlto. "make man" (and thus make doc)
-   requires both.
+   requires asciidoc, but not xmlto. "make man" requires either
+   Asciidoctor or asciidoc+xmlto. "make doc" requires both asciidoc
+   and xmlto.
 
    "make install-doc" installs documentation in man format only; there
    are also "make install-man", "make install-html" and "make
@@ -201,6 +202,11 @@ Issues of note:
    use Asciidoctor (requires Ruby) by passing USE_ASCIIDOCTOR=YesPlease
    to make. You need at least Asciidoctor version 1.5.
 
+   You can also do "make man" using Asciidoctor's manpage backend in
+   place of xmlto by passing USE_ASCIIDOCTOR_MANPAGE=YesPlease. Version
+   2.0 or later is highly recommended, as these version properly handle
+   apostrophes.
+
    There are also "make quick-install-doc", "make quick-install-man"
    and "make quick-install-html" which install preformatted man pages
    and html documentation. To use these build targets, you need to
-- 
2.25.1