[Java][FlexBuffers] Improve documentation for FlexBuffers in Java. (#5506)

Also add a FlexBuffer constructor to simplify usage

docs/source/FlexBuffers.md

@@ -29,9 +29,7 @@ FlexBuffers is still slower than regular FlatBuffers though, so we recommend to
 only use it if you need it.
 
 
-# Usage
-
-This is for C++, other languages may follow.
+# Usage in C++
 
 Include the header `flexbuffers.h`, which in turn depends on `flatbuffers.h`
 and `util.h`.
@@ -122,6 +120,46 @@ map["unknown"].IsNull();  // true
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 
+# Usage in Java
+
+Java implementation follows the C++ one, closely.
+
+For creating the equivalent of the same JSON `{ vec: [ -100, "Fred", 4.0 ], foo: 100 }`,
+one could use the following code:
+
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~{.java}
+FlexBuffersBuilder builder = new FlexBuffersBuilder(ByteBuffer.allocate(512),
+		                                                FlexBuffersBuilder.BUILDER_FLAG_SHARE_KEYS_AND_STRINGS);
+int smap = builder.startMap();
+int svec = builder.startVector();
+builder.putInt(-100);
+builder.putString("Fred");
+builder.putFloat(4.0);
+builder.endVector("vec", svec, false, false);
+builder.putInt("foo", 100);
+builder.endMap(null, smap);
+ByteBuffer bb = builder.finish();
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Similarly, to read the data, just:
+
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~{.java}
+FlexBuffers.Map map = FlexBuffers.getRoot(bb).asMap();
+map.size();  // 2
+FlexBuffers.Vector vec = map.get("vec").asVector();
+vec.size();  // 3
+vec.get(0).asLong();  // -100;
+vec.get(1).asString();  // "Fred";
+vec.get(1).asLong();  // 0 (Number parsing failed).
+vec.get(2).asFloat();  // 4.0
+vec.get(2).asString().isEmpty();  // true (Wrong Type).
+vec.get(2).asString();  // "" (This still works though).
+vec.get(2).toString();  // "4.0" (Or have it converted).
+map.get("foo").asUInt();  // 100
+map.get("unknown").isNull();  // true
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+
 # Binary encoding
 
 A description of how FlexBuffers are encoded is in the

docs/source/doxyfile

@@ -778,7 +778,7 @@ INPUT = "FlatBuffers.md" \
         "../../php/FlatbufferBuilder.php" \
         "../../net/FlatBuffers/FlatBufferBuilder.cs" \
         "../../include/flatbuffers/flatbuffers.h" \
-        "../../go/builder.go"
+        "../../go/builder.go" \
         "../../rust/flatbuffers/src/builder.rs"
 
 # This tag can be used to specify the character encoding of the source files

docs/source/doxygen_layout.xml

@@ -48,7 +48,7 @@
       <tab type="user" url="@ref flatbuffers_guide_use_rust"
           title="Use in Rust"/>
       <tab type="user" url="@ref flexbuffers"
-          title="Schema-less version"/>
+          title="FlexBuffers (Schema-less version)"/>
       <tab type="usergroup" url="" title="gRPC">
         <tab type="user" url="@ref flatbuffers_grpc_guide_use_cpp"
             title="Use in C++"/>