refactor(javac): ale_java_javac_sourcepath variable supports String and List types

doc/ale-java.txt

@@ -69,18 +69,27 @@ g:ale_java_javac_options                             *g:ale_java_javac_options*
 
 g:ale_java_javac_sourcepath                       *g:ale_java_javac_sourcepath*
                                                   *b:ale_java_javac_sourcepath*
-  Type: |List|
+  Type: |String| or |List|
   Default: `[]`
 
-  This variable can set multiple source code paths, the source code path is a
-  relative path (relative to the project root directory).
+This variable can set multiple source code paths, the source code path is a
+relative path (relative to the project root directory).
 
-  Example:
-  >
-    let g:ale_java_javac_sourcepath = [
-      \ 'build/generated/source/querydsl/main',
-      \ 'target/generated-sources/source/querydsl/main'
-    \ ]
+Example:
+
+String type:
+Note that the unix system separator is a colon(`:`) window system
+is a semicolon(`;`).
+>
+  let g:ale_java_javac_sourcepath = 'build/gen/source/xx/main:build/gen/source'
+<
+List type:
+>
+  let g:ale_java_javac_sourcepath = [
+    \ 'build/generated/source/querydsl/main',
+    \ 'target/generated-sources/source/querydsl/main'
+  \ ]
+<
 
 
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