[libc][docs] Add a section about allocations and deallocations to the style doc.

[sivachandra]

Summary

Fixes #59277 - The main part of that bug has already been addressed. This commit
just adds documentation.

libc/docs/dev/code_style.rst

 The libc developers should, and are encouraged to, use assertions freely in
 the libc runtime code. However, the assertion should be listed via the macro
 ``LIBC_ASSERT`` defined in ``src/__support/libc_assert.h``. This macro can be
 used from anywhere in the libc runtime code. Internally, all it does is to
 print the assertion expression and exit. It does not implement the semantics
 of the standard ``assert`` macro. Hence, it can be used from any where in the
 libc runtime code without causing any recursive calls or chicken-and-egg
 situations.
+
+Allocations in the libc runtime code
+====================================
+
+Some libc functions allocate memory. For example, the ``strdup`` function
+allocates new memory into which the input string is duplicated. Allocations
+are typically done by calling a function from the ``malloc`` family of
+functions. Such functions can fail and return an error value to indicate
+allocation failure. To conform to standards, the libc should handle
+allocation failures gracefully and surface the error conditions to the user
+code as appropriate. Since LLVM's libc is implemented in C++, we want
+allocations and deallocations to employ C++ operators ``new`` and ``delete``
+as they implicitly invoke constructors and destructors respectively. However,
+if we use the default ``new`` and ``delete`` operators, the libc will end up
+depending on the C++ runtime. To avoid such a dependence, and to handle
+allocation failures gracefully, we use special ``new`` and ``delete`` operators
+defined in
+`src/__support/CPP/new.h <https://github.com/llvm/llvm-project/blob/main/libc/src/__support/CPP/new.h>`_.
+Allocations and deallocations using these operators employ a pattern like
+this:
+
+.. code-block:: c++
+
+   #include "src/__support/CPP/new.h"
+
+   ...
+
+     __llvm_libc::AllocChecker ac;
+     auto *obj = new (ac) Type(...);
+     if (!ac) {
+       // handle allocator failure.
+     }
+     ...
+     delete obj;
+
+The only exception to using the above pattern is if allocating using the
+``realloc`` function is of value. In such cases, prefer to use only the
+``malloc`` family of functions for allocations and deallocations. Allocation
+failures will still need to be handled gracefully. Further, keep in mind that
+these functions do not call the constructors and destructors of the
+allocated/deallocated objects. So, use these functions carefully and only
+when it is absolutely clear that constructor and desctructor invocation is
+not required.