Clarify the required steps to build the documentation

Previously we didn't mention that the project must have been built
locally first (to get the sources for javadoc generation for instance).

This commit clarifies that.

Closes gh-10086

README.adoc

@@ -121,6 +121,13 @@ another project just mention that in your pull request.)_
 
 === Building reference documentation
 
+First of all, make sure you have built the project:
+
+[indent=0]
+----
+	$ ./mvnw clean install
+----
+
 The reference documentation requires the documentation of the Maven plugin to be
 available so you need to build that first since it's not generated by default.
 
@@ -129,8 +136,9 @@ available so you need to build that first since it's not generated by default.
 	$ ./mvnw clean install -pl spring-boot-tools/spring-boot-maven-plugin -Pdefault,full
 ----
 
-The documentation also includes auto-generated information about the starters. To
+The documentation also includes auto-generated information about the starters. You might
-allow this information to be collected, the starter projects must be built first:
+have that in your local repository already (per the first step) but if you want to refresh
+it:
 
 [indent=0]
 ----