docs: document the node_context_data attribute

It can be useful if you need to turn on/off stamping per-target.
`java_binary` gives a dedicated stamp attribute for this purpose

Fixes bazel-contrib#1693

internal/pkg_npm/pkg_npm.bzl

@@ -85,7 +85,13 @@ PKG_NPM_ATTRS = {
     "node_context_data": attr.label(
         default = "@build_bazel_rules_nodejs//internal:node_context_data",
         providers = [NodeContextInfo],
-        doc = "Internal use only",
+        doc = """Provides info about the build context, such as stamping.
+        
+        By default it reads from the bazel command line, such as the `--stamp` argument.
+        Use this to override values for this target, such as enabling or disabling stamping.
+        You can use the `node_context_data` rule in `@build_bazel_rules_nodejs//internal/node:context.bzl`
+        to create a NodeContextInfo.
+        """,
     ),
     "package_name": attr.string(
         doc = """Optional package_name that this npm package may be imported as.""",

packages/rollup/rollup_bundle.bzl

@@ -156,7 +156,13 @@ Either this attribute or `entry_point` must be specified, but not both.
     "node_context_data": attr.label(
         default = "@build_bazel_rules_nodejs//internal:node_context_data",
         providers = [NodeContextInfo],
-        doc = "Internal use only",
+        doc = """Provides info about the build context, such as stamping.
+        
+        By default it reads from the bazel command line, such as the `--stamp` argument.
+        Use this to override values for this target, such as enabling or disabling stamping.
+        You can use the `node_context_data` rule in `@build_bazel_rules_nodejs//internal/node:context.bzl`
+        to create a NodeContextInfo.
+        """,
     ),
     "output_dir": attr.bool(
         doc = """Whether to produce a directory output.