doc: do not generate documentation by default

Generate the documentation as part of install and explicit rules.
Update the documentation.

Ref: pmem/issues#216

Makefile

@@ -34,6 +34,8 @@
 #
 # Use "make" to build the library.
 #
+# Use "make doc" to build documentation.
+#
 # Use "make test" to build unit tests.
 #
 # Use "make check" to run unit tests.
@@ -78,7 +80,9 @@ rpm dpkg: override prefix=/usr
 
 all:
 	$(MAKE) -C src $@
-	$(MAKE) -C doc $@
+
+doc:
+	$(MAKE) -C doc all
 
 clean:
 	$(MAKE) -C src $@
@@ -132,4 +136,4 @@ install uninstall:
 	$(MAKE) -C doc $@
 
 .PHONY: all clean clobber test check cstyle check-license install uninstall\
-	source rpm dpkg pkg-clean pcheck check-remote format $(SUBDIRS)
+	source rpm dpkg pkg-clean pcheck check-remote format doc $(SUBDIRS)

README.md

@@ -115,6 +115,13 @@ DESTDIR variable, e.g.:
 ```
 This will install files to /tmp/usr/lib, /tmp/usr/include /tmp/usr/share/man.
 
+The man pages (groff files) are generated as part of the `install` rule. To
+generate the documentation separately, run:
+```
+	$ make doc
+```
+**DEPENDENCIES:** pandoc
+
 To install a complete copy of the source tree to $(DESTDIR)/nvml:
 ```
 	$ make source DESTDIR=some_path