Update top-level .gitignore.

Add a comment explaining our policy that we don't put excludes
for nonstandard tools into committed .gitignore files.

Also, remove the entries about libraries with .sl extensions,
since that convention was only used on now-desupported HP-UX.

Discussion: https://postgr.es/m/CAHxW8BAiyPwfXbN813GhorQozwMBs4f3DTxLkKNxiGQuJuw4Vw@mail.gmail.com

.gitignore

@@ -1,3 +1,7 @@
+# This file contains ignores for build artifacts from standard builds.
+# Auxiliary files from local workflows, your preferred editor, etc. should
+# be ignored locally using $GIT_DIR/info/exclude or ~/.gitexclude.
+
 # Global excludes across all subdirectories
 *.o
 *.obj
@@ -6,10 +10,6 @@
 *.so.[0-9]
 *.so.[0-9].[0-9]
 *.so.[0-9].[0-9][0-9]
-*.sl
-*.sl.[0-9]
-*.sl.[0-9].[0-9]
-*.sl.[0-9].[0-9][0-9]
 *.dylib
 *.dll
 *.exp