docs: meson: Change what 'docs' target builds

This undoes the change in what the 'docs' target builds 969509c3f2. Tom was
concerned with having a target to just build the html docs, which a prior
commit now provided explicitly.

A subsequent commit will overhaul the documentation for the documentation
targets.

While at it, move all target in doc/src/sgml/Makefile up to just after the
default "html" target, and add a comment explaining "all" is *not* the default
target.

Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://postgr.es/m/20230209203855.njrepiupc3rmehfw@awork3.anarazel.de
Discussion: https://postgr.es/m/20231103163848.26egkh5qdgw3vmil@awork3.anarazel.de

doc/src/sgml/Makefile

@@ -15,6 +15,8 @@
 # Make "html" the default target, since that is what most people tend
 # to want to use.
 html:
+# Note that all is *not* the default target in this directory
+all: html man
 
 # We don't need the tree-wide headers or install support here.
 NO_GENERATED_HEADERS=yes
@@ -25,8 +27,6 @@ top_builddir = ../../..
 include $(top_builddir)/src/Makefile.global
 
 
-all: html man
-
 
 ifndef DBTOEPUB
 DBTOEPUB = $(missing) dbtoepub

doc/src/sgml/docguide.sgml

@@ -442,7 +442,7 @@ LOGLEVEL=-Dorg.apache.commons.logging.simplelog.defaultlog=WARN
    <para>
     To build just the <acronym>HTML</acronym> version of the documentation:
 <screen>
-<prompt>build$ </prompt><userinput>ninja docs</userinput>
+<prompt>build$ </prompt><userinput>ninja html</userinput>
 </screen>
     To build all forms of the documentation:
 <screen>

doc/src/sgml/meson.build

@@ -235,9 +235,9 @@ if docs_dep.found()
   alias_target('man', man)
   alias_target('install-man', install_doc_man)
 
-  # even though we don't want to build man pages as part of 'docs', we do want
-  # to install them as part of install-docs
+  # built and installed as part of the the docs target
   installdocs += install_doc_man
+  docs += man
 endif