std.Build.Step.Compile: getEmittedDocs API enhancements

 * Allow calling it multiple times.
 * Rename it. Sorry, this is to coincide with #16353.

Author: andrewrk

build.zig

@@ -61,7 +61,7 @@ pub fn build(b: *std.Build) !void {
     autodoc_test.overrideZigLibDir("lib");
     autodoc_test.emit_bin = .no_emit; // https://github.com/ziglang/zig/issues/16351
     const install_std_docs = b.addInstallDirectory(.{
-        .source_dir = autodoc_test.getOutputDocs(),
+        .source_dir = autodoc_test.getEmittedDocs(),
         .install_dir = .prefix,
         .install_subdir = "doc/std",
     });

lib/std/Build/Step/Compile.zig

@@ -1004,8 +1004,8 @@ pub fn getOutputPdbSource(self: *Compile) FileSource {
     return .{ .generated = &self.output_pdb_path_source };
 }
 
-pub fn getOutputDocs(self: *Compile) FileSource {
-    assert(self.generated_docs == null); // This function may only be called once.
+pub fn getEmittedDocs(self: *Compile) FileSource {
+    if (self.generated_docs) |g| return .{ .generated = g };
     const arena = self.step.owner.allocator;
     const generated_file = arena.create(GeneratedFile) catch @panic("OOM");
     generated_file.* = .{ .step = &self.step };