Add "Remarks" headings for warning references in range [PRJ0018, PRJ0049]

Author: Rageking8

docs/error-messages/tool-errors/project-build-warning-prj0018.md

@@ -10,6 +10,8 @@ ms.assetid: 1a3d9e40-6a35-4a74-b6ba-8079f7a82217
 
 > The following environment variables were not found:
 
+## Remarks
+
 An environment variable is not defined. This error lists the environment variables that were not defined.
 
 See [Macros for Custom Build Commands](../../build/reference/common-macros-for-build-commands-and-properties.md) for information on macros in the build process.

docs/error-messages/tool-errors/project-build-warning-prj0029.md

@@ -10,6 +10,8 @@ ms.assetid: f02c09c6-09f3-4d44-8cd4-9a25336be1ea
 
 > The 'Outputs' property for the project-level custom build step is not set. The custom build step will be skipped.
 
+## Remarks
+
 A custom build step was not executed because no output was specified.
 
 To resolve this error, do one the following:

docs/error-messages/tool-errors/project-build-warning-prj0041.md

@@ -10,6 +10,8 @@ ms.assetid: dc9f4cf9-6bd5-4222-89e8-7802a59bb96b
 
 > Cannot find missing dependency 'dependency' for file 'file'. Your project may still build, but may continue to appear out of date until this file is found.
 
+## Remarks
+
 A file (resource file or .idl/.odl file, for example, contained an include statement that the project system could not resolve.
 
 Because the project system does not process preprocessor statements (#if, for example), the offending statement may not actually be part of the build.

docs/error-messages/tool-errors/project-build-warning-prj0042.md

@@ -10,6 +10,8 @@ ms.assetid: 682c9999-6f85-409f-b102-00c93243f74f
 
 > The 'Outputs' property for the custom build step for file '*file*' is not set. The custom build step will be skipped.
 
+## Remarks
+
 A custom build step was not executed because no output was specified.
 
 To resolve this error, do one the following:

docs/error-messages/tool-errors/project-build-warning-prj0049.md

@@ -9,6 +9,8 @@ ms.assetid: 8b38afa1-e080-4efd-ae89-776cfd044413
 
 > Referenced target '\<Reference>' requires .NET Framework \<MinFrameworkVersion> and will fail to run on this project's target framework
 
+## Remarks
+
 Applications created by using Visual Studio 2008 can specify which version of the .NET Framework they should target. If you add a reference to an assembly or project that depends on a version of the .NET Framework that is later than the targeted version, you will get this warning at compile time.
 
 ### To correct this warning