Genex: Add a test for the LINK_ONLY expression.

Commit 239b0c6b (Don't add invalid content to static lib
INTERFACE_LINK_LIBRARIES., 2013-10-20) extended a test which
excercised the logic of the LINK_ONLY generator expression.

Commit ef10b87c (CMP0022: Plain target_link_libraries must populate
link interface, 2013-11-02) removed the instance of LINK_ONLY which
was excercised by that test.

Add a new test which excercises the other instance of LINK_ONLY by
setting the CMP0022 policy to NEW and consuming the contents of the
INTERFACE_LINK_LIBRARIES target property.
Disallow invalid target names (#13140)

Exclude Borland and NMake from the CMP0037 test. They do not accept
the colon in a target name.
Disallow link-to-self (#13947).
Disallow linking to utility targets (#13902).
Help: Format execute_process command documentation

Use a code-block for the signature.  Format the options as a definition
list.
Help: Clarify execute_process COMMAND argument behavior

State in the execute_process how COMMAND arguments are passed to
the child.  Add a note at the end about how the command differs
from add_custom_*().

Suggested-by: Alan W. Irwin
Help: Add cmake-language.7 manual
Help: Reference cmake-language.7 comment docs from cmake-developer.7

The cmake-language.7 manual now documents comment syntax.  Reference it
from cmake-developer.7 when discussing how to add comment-enclosed
documentation to modules.
Merge topic 'doc-execute_process'

3bb6266 Help: Clarify execute_process COMMAND argument behavior
ea6b656 Help: Format execute_process command documentation
Merge topic 'doc-cmake-language'

79fd233 Help: Reference cmake-language.7 comment docs from cmake-developer.7
a931ee4 Help: Add cmake-language.7 manual
Merge topic 'tll-target-policies'

596b2a8 Disallow linking to utility targets (#13902).
301bb5c Disallow link-to-self (#13947).
05f5fde Disallow invalid target names (#13140)
Merge topic 'test-LINK_ONLY-expression'

53b10fd Genex: Add a test for the LINK_ONLY expression.